a method and apparatus for measuring lead impedance during preselected test mode operation of an implantable body tissue stimulator . the analysis circuitry is periodically triggered into operation , such as on each reprogramming by the physician or periodically as a function of elapsed time or number of stimulation events counted from the preceding measurement . the actual lead impedance , measured from the output circuit from the body tissue - stimulator pulse generator , and taking into account impedance of the interconnection between the lead connector pin and the pulse generator connector block , the lead electrical conductor and its connections with the electrode and the connector pin and the electrode - tissue interface , is calculated as a function of the ratio of the elapsed times that it takes to discharge a capacitor from a first reference voltage to a second reference voltage through a precision resistor and through the lead impedance itself the calculated lead impedance may be stored in memory with a suitable time tag , employed to automatically effect a change in operating modes or change a lead and electrode selection , if measured lead impedance falls outside normal high and low impedance boundary values . in the pacing context , calculated lead impedance may be employed to adjust sense amplifier sensitivity and pacing output pulse parameters . the method and apparatus may also be employed to calculate cardioversion / defibrillation lead impedance through selective partial discharge of high voltage output capacitors .