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The photoprotein aequorin was injected into superficial cells of ferret papillary muscles. Tension and aequorin light (a function of intracellular [Ca2+]) were monitored. Increasing intracellular Ca concentration ([Ca2+]i), either by decreasing extracellular Na, or by inhibiting the Na pump with strophanthidin, produced spontaneous oscillations of [Ca2+]i and tension. Fourier analysis showed that these oscillations had frequencies of up to 3-4 Hz. If the muscle was stimulated in these conditions the Ca transient associated with the twitch was followed by a series of damped oscillations of [Ca2+]i which were accompanied by after-contractions. Under a given set of conditions the frequency of the stimulated oscillations was similar to that of the spontaneous oscillations. Manoeuvres which increase [Ca2+]i increased the frequency of both spontaneous and stimulated oscillations. Drugs which inhibit the function of the sarcoplasmic reticulum (caffeine and ryanodine) abolished both stimulated and spontaneous oscillations. The spontaneous oscillations during a Na-free contracture were unaffected by the Ca channel blocker D-600. When repetitive stimulation was begun the frequency and magnitude of the stimulated oscillations increased over several minutes. Increasing the frequency of stimulation increased the magnitude of the stimulated oscillations. It is concluded that the spontaneous oscillations of [Ca2+]i may be due to oscillatory Ca release from the sarcoplasmic reticulum. The similar properties of the spontaneous and stimulated oscillations suggest that the latter may be due to a synchronization of the former.