The membrane potential s and isometric tension of canine ventricular muscle fibers in the K+-free, Ca2+-rich solution developing oscillatory afterpotentials (OAPs) were recorded simultaneously. The OAPs were always accompanied by aftercontractions (ACs). Both activities appeared with almost a similar frequency and timing. The amplitude of the OAPs, ACs and twitch tension were dependent on the basic cycle length of the train as well as on the number of impulses. The manner of this dependence was similar in the OAPs and ACs but showed a slight difference in the twitch tension. The increase in the [Ca2+]0 and the application of isoproterenol caused augmentation of the OAPs during the application. By washing out the drug, the OAPs and ACs quickly recovered to the control level but the twitch tension did not. The application of 2 mM K+ or Rb+ quickly and reversibly abolished the OAPs and ACs with a marked suppression of the twitch tension. One mM of caffeine eliminated the oscillations of both the membrane potentials and tension. The wash-out of caffeine brought about a transient increase in the OAPs and ACs. Two or five mM of procaine caused a complete elimination of the OAPs and ACs. These results suggest that the OAPs are formed by a process related to the cyclic release of Ca2+ from the sarcoplasmic reticulum.