The photoprotein aequorin was microinjected into cells on the surface of ferret papillary muscles. Tension and aequorin light, a function of intracellular Ca concentration ([Ca2+]i), were monitored. The preparations were exposed to increased concentrations of extracellular Ca ([Ca2+]o). Small increases in [Ca2+]o led to the usual increase in both the systolic light signal and the developed tension. However, high [Ca2+]o led to a fall in developed tension known as Ca overload. This fall of tension was seen at lower [Ca2+]o if the experiment was performed in the presence of strophanthidin. In conditions of Ca overload, the systolic light signal was either similar in amplitude or larger than the systolic light observed under conditions which did not lead to Ca overload. Oscillations of diastolic light were invariably present under conditions of Ca overload. These oscillations were accompanied by after-contractions which were small in relation to the magnitude of the aequorin light oscillations. During Ca overload, the variance of the amplitude of the systolic light signal was greater than could be accounted for by the random nature of the arrival of photons. Small systolic light signals occurred when there had been an oscillation of light in the diastolic period immediately preceding the systolic light signal. Large systolic light signals occurred when the preceding period (approximately 1 s) was free of oscillations of light. These observations suggest that if the sarcoplasmic reticulum (s.r.) has spontaneously released its contents of Ca as shown by a diastolic [Ca2+]i oscillation, then a stimulated systolic Ca signal occurring within about the next second is smaller, possibly because it takes longer than this period for the s.r. to reload with Ca. If this process occurs randomly in the various cells of the preparation, developed tension will be reduced because those cells which have a small Ca release will act as a compliance in series with cells which have a large Ca release.