Those who live in other people's skins don't need to worry about their own aging flesh. That, at least, would seem to be the comforting case with Kathy Najimy and Mo Gaffney, currently reviving some of the skits that made them comedy cult stars in the 1980's.

Their affectionate, relaxed show, which opened last night at Second Stage Theater, is called ''Afterbirth: Kathy and Mo's Greatest Hits.'' And whether they're portraying teenage girls pondering the lessons of ''West Side Story'' or feisty senior citizens in a women's studies class, Ms. Najimy and Ms. Gaffney still seem just as adolescent or geriatric as the occasion demands.

Middle-aged audience members who have been rattled by seeing rock idols of their youth transformed into Madame Tussaud-ish creatures will receive no similar shock here. While these performers (now in their 40's) have included a new routine devoted to the demoralizing, all-pervasive lust for cosmetic surgery among women, they both look their ages and consistently elude such categorization. If some of their sketches belong firmly to the past -- like a send-up of feminine hygiene ads that brings to mind early ''Saturday Night Live'' -- the empathy with which they invest their alter egos remains very much of the present.

''Afterbirth,'' directed by Mark Brokaw, reunites Ms. Najimy and Ms. Gaffney for a limited engagement, through July 11, as part of the 25th-anniversary season of Second Stage, the theater company that presented ''The Further Adventures of Kathy and Mo'' in 1986. Both women, who appeared together in two specials for HBO, have gone on to independent careers in film and television. This is their first joint appearance in New York in 15 years.

Though their feminist-flavored humor developed an ardent following soon after they began their act, the team of Kathy and Mo has never really qualified as cool. Coolness usually implies a mordant distance, a flip hipness, and Ms. Najimy and Ms. Gaffney are nothing if not warm. This is true even when they embody characters with whom they would appear to have little affinity, like the blue-collar anti-abortion demonstrator at a birth clinic portrayed by Ms. Gaffney, or the drunken old Texas barfly whom Ms. Najimy becomes in what is probably the show's best-known sketch.

Like Lily Tomlin, an obvious inspiration, they channel women and men of different ages, political persuasions and classes to create a running social commentary, generally managing to do so without condescension. Even when proselytizing for gay marriage and racial tolerance (in a recurring bit where they appear as angels creating the human race), they are far gentler than the generation of more frenetic, confrontational comedians that has succeeded them.

Their writing, for the most part, isn't deeply insightful or riotously funny in itself. Only a new sketch about a support group for the absent mothers in Walt Disney cartoons like ''Bambi'' and ''The Little Mermaid'' actually reads as well as it plays, if not better. What makes these women appealing are the humanizing physical quirks they bring to characterizations that seem to have been arrived at less from the outside than from within.

Their superficial personae are, as in most comedy teams, antithetical, with Ms. Najimy doing emotionally wet and expansive to Ms. Gaffney's dry and deadpan. But they share an embracing attitude toward the bewildered and frustrated souls they incarnate. You have little doubt that Ms. Najimy and Ms. Gaffney actually like Hank and Karen Sue, those unhappy lushes who enact a nightly ritual of empty flirtation in a country-western bar.

That's the sketch in which Ms. Najimy, as Hank, dangles a cigarette from her mouth as if it were the central tooth in a grotesque overbite. You can sense the nostalgic rush this bit of shtick inspires in Kathy and Mo aficionados, just as the first notes of ''Landslide'' might induce heavy, contented sighs at a Fleetwood Mac concert.

Theatergoers new to these comedians are unlikely to experience quite the same satisfaction. But the generosity of spirit in evidence here creates a soothing retreat from the harsh comedy of humiliation that rules the nightclubs and airwaves.


Kathy and Mo's Greatest Hits

By Mo Gaffney and Kathy Najimy; directed by Mark Brokaw; sets by Allen Moyer; costumes by Linda Ross; lighting by Mary Louise Geiger; sound by Jon Gottlieb; production stage manager, Pam Edington; assistant stage manager, Gerald Cosgrove; associate artistic director, Christopher Burney; general manager, C. Barrack Evans. Presented by Second Stage Theater, Carole Rothman, artistic director; Timothy J. McClimon, executive director. At 307 West 43rd Street, Clinton.

WITH: Mo Gaffney and Kathy Najimy.

Photo: Kathy Najimy, left, and Mo Gaffney as Hank and Karen Sue in an ''Afterbirth'' sketch. (Photo by Michael Lamont)