This paper outlines a simple method of estimating both the Ca-buffering properties of the cytoplasm and the time-course of changes of sarcoplasmic reticulum (s.r.) Ca concentration during systole. The experiments were performed on voltage-clamped ferret single ventricular myocytes loaded with the free acid of fluo-3 through a patch pipette. The application of caffeine (10 mM) resulted in a Na-Ca exchange current and a transient increase of the free intracellular Ca concentration ([Ca²⁺]_i). The time-course of change of total Ca in the cell was obtained by integrating the current and this was compared with the measurements of [Ca²⁺]_i to obtain a buffering curve. This could be fit with a maximum capacity for the intrinsic buffers of 114±18 µmol l^–1 and K _d of 0.59±0.17 µM (n=8). During the systolic rise of [Ca²⁺]_i, the measured changes of [Ca²⁺]_i and the buffering curve were used to calculate the magnitude and time-course of the change of total cytoplasmic Ca and thence of both s.r. Ca content and Ca release flux. This method provides a simple and reversible mechanism to measure Ca buffering and the time-course of both total cytoplasmic and s.r. Ca.