It is well known that the strength of postextrasystolic potentiation (PESP) is dependent on the prematurity of the ectopic beat, though the fundamental mechanism of the potentiation is still obscure. In this study, the effect of a resting interval on the strength of PESP was investigated in isolated papillary muscles of guinea pigs in the presence or absence of caffeine, which inhibits the functions of sarcoplasmic reticulum (SR). PESP of a fixed coupling interval increased and then decreased as the resting interval was prolonged. The maximum of PESP was obtained at a resting interval of 3 to 4 sec. The dependency of PESP on a coupling interval was decreased considerably by 5 x 10(-4) M caffeine and removed completely by 10(-2) M caffeine. Although 5 x 10(-4) M caffeine decreased the degree of contraction of postextrasystole, the maximum contraction of postextrasystole was still obtained at a resting interval of 3 to 4 sec. After the application of 10(-2) M caffeine, the postextrasystolic contraction gradually declined as the resting interval was prolonged. We conclude that SR Ca release contributes largely to a mechanism of PESP and increases in contribution as the coupling interval of an extrastimulation shortens, and that the optimal resting interval is determined by a balance between the activity of SR function and the activity of the sarcolemmal Ca extrusion mechanism.