Isometric force and membrane action potential were recorded simultaneously in rabbit papillary muscles (36.5 degrees--37.5 degrees C). One to three test stimuli were given at various intervals (0.20--10.0 s) after a series of control contractions at constant stimulation intervals (1.0--1.5 s). Optimum peak force always occurred when the preceding test interval was 0.80 s. When this interval was greater than 0.80 s, time to peak force was a linear function of the action potential duration. Furthermore, under these conditions the action potential duration (AP1) and peak force (F1) of the test contraction could be used to predict peak force (F2) of the subsequent contraction elicited after a fixed interval (0.80--1.50 s) according to the equation (regression plane): F2 = BAPAP1 + BFF1 + A. Constants BAP and BF are interpreted to provide information about calcium influx during the action potential and of the recirculation of calcium between contractions, respectively. F2 deviated towards higher values than predicted from the equation when the preceding test contraction was triggered to occur at an interval less than 0.80 s. This may be due to an intensified calcium transport into the cell during the action potential after these short intervals. The action potential duration was inversely related to both the inotropic state of the muscle (representing a feed-back mechanism) and the preceding stimulation interval.