the cgmp pde inhibitors useful in this invention as cgmp elevators may be widely chosen from among any of those already known to the art or subsequently discovered and / or hereafter developed . suitable cgmp pde inhibitors include those disclosed in any of the following u . s . patents , all of which are incorporated herein by reference : a griseolic acid derivative as disclosed in any of u . s . pat . nos . 4 , 634 , 706 , 4 , 783 , 532 , 5 , 498 , 819 , 5 , 532 , 369 , 5 , 556 , 975 , and 5 , 616 , 600 ; a fused pyrimidine derivative as disclosed in u . s . pat . no . 5 , 047 , 404 ; a condensed pyrimidine derivative as disclosed in u . s . pat . no . 5 , 075 , 310 ; an imidazoquinoxalinone derivative or its aza analog as disclosed in u . s . pat . nos . 5 , 055 , 465 and 5 , 166 , 344 ; a 4 - aminoquinazoline derivative as disclosed in u . s . pat . no . 5 , 436 , 233 or 5 , 439 , 895 ; a polycyclic guanine derivative as disclosed in u . s . pat . no . 5 , 393 , 755 ; a nitogenous heterocyclic compound as disclosed in u . s . pat . no . 5 , 576 , 322 ; a quinazoline derivative as disclosed in u . s . pat . no . 4 , 060 , 615 ; and other disclosures of cgmp pde inhibitors include the following , all of which are herein incorporated by reference : the potassium channel opening activity of a compound , and therefore its suitability for use in the present invention , can be determined using a number of conventional assays in vitro . potassium channel openers exhibit potassium channel opening activity in relation to the plasma lemma membrane as demonstrated by their influence at concentrations in the region of 1 to 500 nm on various smooth muscle preparations in accordance with or analogous to the methods described in quast , brit . j . pharmac ., 91 , 569 - 578 ( 1987 ). the cgmp pde inhibition of a compound can also be determined by standard assays known to the art , for example as disclosed in u . s . pat . no . 5 , 250 , 534 , incorporated herein by reference . compounds which are selective inhibitors of cgmp pde relative to camp pde are preferred , and determination of such compounds is also taught in u . s . pat . no . 5 , 250 , 534 . particularly preferred are compounds which selectively inhibit the pde v isoenzyme , as disclosed in the aforementioned pct / ep94 / 01580 , published as wo 94 / 28902 . as disclosed above , individual compounds of the combinations useful in this invention will generally be administered separately , each by its own customary and known route , and in certain cases the routes of administration may be different . in a preferred embodiment , administration will generally be timed so that both the potassium channel opener and the cgmp elevator both coincide , or nearly coincide , in reaching their maximum pharmacokinetic effect . the routes of administration can be any of those known to the art such as oral , parenteral via local injection intracavernosally or intraurethrally , or transdermal as by applying the active component in a gel or other such formulation topically to the penis . each component can be formulated as known in the art , usually together with a pharmaceutically acceptable vehicle , diluent or carrier , for example as a tablet , capsule , lozenge , troche , elixir , solution , or suspension for oral administration , in a suitable injectable vehicle for parenteral administration , or as a lotion , ointment or cream for topical application . in a preferred embodiment , the cgmp elevator and the potassium channel opener are each co - administered orally , together or separately . the exact dose of each component administered will , of course , differ depending on the specific components prescribed , on the subject being treated , on the severity of the impotence or of the female sexual dysfunction , on the manner of administration and on the judgment of the prescribing physician . thus , because of patient - to - patient variability , the dosages given below are a guideline and the physician may adjust doses of the compounds to achieve the treatment that the physician considers appropriate for the patient , male or female . in considering the degree of treatment desired , the physician must balance a variety of factors such as he age of the patient and the presence of other diseases or conditions ( e . g ., ardiovascular disease ). in general , the cgmp elevator will be administered in a range of from 0 . 5 to 200 mg per day , preferably 5 to 125 mg per day , more preferably 25 - 100 mg per day . the potassium channel opener will generally be administered in an amount of from 0 . 01 mg to 50 mg per day , preferably from 0 . 5 to 10 mg per day . if the cgmp pde elevator is a prostaglandin , it is generally administered intracavemosally by injection in an amount of from 1 ng to 100 μg or intraurethrally in an amount of 100 μg to 2 mg per day . generally , the injected amount is in a volume which usually will not exceed 1 ml . the carrier or diluent is typically sterile physiological saline or another physiologically acceptable salt solution . oral administration of prostaglandins is also feasible . japanese journal of urology , 83 ( 10 ): 1655 - 1661 , ( 1992 ). as previously disclosed , the combination of cgmp pde elevator and potassium channel opener can be administered as a composition . thus , the compounds of this invention can be administered together in any conventional oral , parenteral , rectal or transdermal dosage form , usually also together with a pharmaceutically acceptable vehicle , diluent or carrier . for oral administration , a pharmaceutical composition can take the form of solutions , suspensions , tablets , pills , capsules , powders , and the like . tablets containing various excipients such as sodium citrate , calcium carbonate and calcium phosphate are employed along with various disintegrants such as starch and preferably potato or tapioca starch and certain complex silicates , together with binding agents such as polyvinylpyrrolidone , sucrose , gelatin and acacia . additionally , lubricating agents such as magnesium stearate , sodium lauryl sulfate and talc are often very useful for tabletting purposes . solid compositions of a similar type are also employed as fillers in soft and hard - filled gelatin capsules ; preferred materials in this connection also include lactose or milk sugar as well as high molecular weight polyethylene glycols . when aqueous suspensions and / or elixirs are desired for oral administration , the compounds of this invention can be combined with various sweetening agents , flavoring agents , coloring agents , emulsifying agents and / or suspending agents , as well as such diluents as water , ethanol , propylene glycol , glycerin and various like combinations thereof . for purposes of parenteral administration , solutions in sesame or peanut oil or in aqueous propylene glycol can be employed , as well as sterile aqueous solutions of the corresponding water - soluble salts . such aqueous solutions may be suitably buffered , if necessary , and the liquid diluent first rendered isotonic with sufficient saline or glucose . these aqueous solutions are especially suitable for intravenous , intramuscular , subcutaneous and intraperitoneal injection purposes . in this connection , the sterile aqueous media employed are all readily obtainable by standard techniques well - known to those skilled in the art . for purposes of transdermal ( e . g ., topical ) administration , dilute sterile , aqueous or partially aqueous solutions ( usually in about 0 . 1 % to 5 % concentration ), otherwise similar to the above parenteral solutions , are prepared . methods of preparing various pharmaceutical compositions with a certain amount of active ingredient are known , or will be apparent in light of this disclosure , to those skilled in this art . for examples of methods of preparing pharmaceutical compositions , see remington &# 39 ; s pharmaceutical sciences , mack publishing company , easter , pa ., 15th edition ( 1975 ). a combination of a potassium channel opener and a cgmp elevator such as a cgmp pde inhibitor can be tested in vivo in either a beagle dog or monkey model . the following description is with respect to monkeys , but those skilled in the art will easily recognize that the test applies equally and can be adapted to beagle dogs . mature adult male monkeys , typically either cercopithecus aethiops ( green monkey ) or macaca fasciculata ( cynomologous ) having a weight range of 4 to 8 kg are used . animals are anesthetized with diazepam ( 2 . 5 mg ), ketamine chloride ( 20 μg / kg i . m . supplemented as appropriate ) and given the test compound ( s ) dissolved in saline intracavernosally ( 0 . 3 ml ). animals are placed supine , the penis stretched out , and a rubber band placed around the root of the base as a tourniquet kept in place for three minutes after the injection . the solution is injected through a 27g needle into one of the corpus cavernosa and 5 , 10 , 25 , 30 , 60 , and 180 minutes later tumescence ( increase in volume ) and rigidity of the penis is estimated visually and by palpitation . to determine the threshold effect using the injectable solution , a series of animals are used covering an appropriate dose range for the test compound or compounds . the combination of a potassium channel opener and cgmp elevator can also be tested clinically , typically orally , in humans as well as in an animal model . each compound is administered singly at different times to a population of male patients , each compound being administered in an amount which produces little or no response , typically less than a 50 % response , as measured by the rigiscan clinical evaluation parameters ( see kaneko et al ., j . urol . 136 , 1026 - 1029 ( 1986 ); and ogric et al ., j . urol ., 154 , 1356 - 1359 ( 1995 ) ) of rigidity and tumescence , in conjunction with the international index of erectile function ( iief ) questionnaire which evaluates patient and partner satisfaction . by administering each compound singly , it is meant that one compound is administered , followed at a later time by the second compound after having allowed an appropriate time for washout of the first compound . after the washout period for each compound administered singly , the compound are co - administered in a manner such that both compounds co - operate pharmacokinetically , preferably such that the peak pharmacokinetic effect due to each coincides . co - administration is evaluated according to the regiscan parameters mentioned above and by iief questionnaires , thereby providing a basis for comparison of the effects of co - administration with that for each single administration and to demonstrate a synergistic effect . the compounds and combinations of this invention generally will be administered in a convenient formulation . the following formulation examples are illustrative only and are not intended to limit the scope of the present invention . in the formulations which follow , “ active ingredient ” means a compound ( s ) of this invention . alternatively , tablets each containing 0 . 25 - 100 mg of active ingredients are made up as follows : the active ingredient , starch , and cellulose are passed through a no . 45 mesh u . s . sieve and mixed thoroughly . the solution of polyvinylpyrrolidone is mixed with the resultant powders which are then passed through a no . 14 mesh u . s . sieve . the granules so produced are dried at 50 °- 60 ° c . and passed through a no . 18 mesh u . s . sieve . the sodium carboxymethyl starch , magnesium stearate , and talc , previously passed through a no . 60 u . s . sieve , are then added to the granules which , after mixing , are compressed on a tablet machine to yield tablets .