The force-interval relationship--the dependence of cardiac contractility on the rate and pattern of stimulation--was evaluated for its potential use in monitoring patients in the period immediately following heart surgery. Six patients were studied for three days after coronary artery bypass grafting. The monitoring instrumentation used during operation included a catheter-tip micromanometer introduced into the left ventricle, a pair of ultrasonic transducers placed on the left ventricular (LV) epicardium to monitor minor-axis dimension, pacing electrodes placed on the right atrium, and systemic arterial and venous catheters. During the experiments, heart rate was controlled by atrial pacing. After every twentieth systole (the control systole), a pause in the heart rate was introduced during which an extrasystole (SE) and a postextrasystole (SPES) were elicited at test intervals tE and tPES, respectively. The intervals between the control systole and test systoles SE and SPES were experimentally manipulated. When the test intervals were increased, the peak first derivative of LV pressure (Pmax) of SE and of SPES increased monophasically. To eliminate the effects of LV end-diastolic volume, we used only SPES and control systoles with the same LV end-diastolic dimension in construction of PES ratio curves (Pmax of SPES/Pmax of the control systole, as a function of tPES). The PES ratio rose monophasically with an increase in tPES; these curves were well fitted by an exponential relationship. The PES ratio exceeded unity at long tPES intervals. This ratio, denoting postextrasystolic potentiation, was inversely dependent on tE. The patients experienced no complications.(ABSTRACT TRUNCATED AT 250 WORDS)