From a total of 51 patients equipped with rate and pulse width adjustable pulse generators (Microlith-P 0505, Microthin-PI 0522) implanted over the last 3 years, 10 (19.6%) showed an unexpected drop in pacemaker pulse rate during pulse width programming. For one of the pulse generators used (Microthin-PI 0522), unexpected rate decrease occurred in 7/13 cases (53.8%). For all except one patient, decrease in pacemaker pulse rate corresponded with the total refractory period of the pulse generator ( 320 ms), at a certain pulse width when rate drop first occurred. In seven of the patients the pulse generator automatic interval was extended from 13 ms to 171 ms beyond the refractory period. In two patients it was necessary to replace the pulse generators. Our study strongly proves that this abnormal pacemaker functioning is a result of sensing of the polarization voltage at the pacemaker electrode/tissue interface and/or the T-wave. The polarization voltage is highly dependent on the total pacemaker electrode/tissue interface impedance. Using typical values for pulse generator output and input resistance and output capacitance, Faraday resistance, Helmholtz capacitance and tissue resistance at the electrode/tissue interface it was shown mathematically that in some cases the polarization voltage alone would be of sufficient amplitude and slew rate for pacemaker inhibition. The study demonstrates an urgent need for change in the filter characteristics by making the pulse generators less sensitive in the low frequency region and reducing the polarization voltage by reducing the output circuit capacitance.