Endocardial electrograms in 77 acute cases (at time of pacemaker implant) and 56 chronic cases (at time of pulse generator replacement, with electrodes in service for at least 6 months) were recorded. The peak-to-peak voltage and rate of rise, or slew rate, were compared acutely and chronically. Both parameters must simultaneously exceed given thresholds to trigger the sensing circuits of implantable pacemakers. The average chronic slew rate was half the average acute value. The maximum slew rate and voltage swing occur during the intrinsic deflection as a virtually straight line segment of the electrogram. The findings are consistent with the predictions of a mathematical model for genesis of the electrogram, and also with a model for explaining acute-to-chronic changes in stimulation current thresholds.