Sarcoplasmic reticulum (SR) vesicles contain an ATP-dependent calcium pump that transports Ca2+ from the external medium into the vesicles against a large Ca2+ gradient. In addition, these vesicles can release Ca2+ by a process that shares many of the characteristics of the calcium pump, suggesting that the calcium pump ATPase protein also participates in calcium efflux. The appearance of a calcium release phase in cardiac SR vesicles is seen in the presence of high external Ca2+. Although this dependence on high Ca2+ resembles that of "aftercontractions" in intact heart muscle, the periodicity of the oscillations in calcium content of cardiac SR vesicles is much slower than that of aftercontractions in the intact myocardium.