The sarcoplasmic reticulum (SR) provides the majority of calcium to the systolic calcium transient. It is this rise in intracellular calcium that causes cardiac systole. The SR is, therefore, important for maintaining cardiac contractility. SR calcium content is replenished by the sacro(endo)plasmic reticulum calcium ATP-ase (SERCA). It might be expected that reducing SERCA activity would result in an equally decreased SR calcium content, impairing contractility. However, Andersson et al. (2009) have shown, in a SERCA2 conditional knock-out mouse model, that reduction of cardiac SERCA2 expression to 5 % of control levels only decreased SR calcium to 38 % of control. We now investigate how acute changes to SERCA activity (kSERCA) affect SR calcium content in the normal ventricular myocyte. Single isolated ventricular myocytes from rats were used and intracellular calcium concentration measured with fluo-5. Experiments were performed at 37 °C, using the perforated patch voltage-clamp technique. Systolic calcium transients were evoked with depolarizing pulses at 0.25 Hz. kSERCA was quantified from the rate constant of decay of the calcium transient. SR calcium content was determined from the integral of the caffeine-evoked Na-Ca exchange current. 1 μM thapsigargin was applied to the cell to slow kSERCA to varying levels. The data reported is expressed as the mean ± SEM. Significance was tested using One Way Repeated Measures ANOVA. Upon repeated application of thapsigargin kSERCA slowed by 65 % (Figure 1) from 7.4 ± 0.8 s-1 to a final value of 2.6 ± 1.5 s-1 (n = 4 – 13 cells, p = 0.001) and SR calcium content decreased by 46 % from 82 ± 5 μM to 44 ± 5 μM (n = 4 – 13 cells, p < 0.001). Using this data we found that SR calcium content was proportional to (SERCA activity)1/m where the mean value for m was 2.59. Therefore, a large reduction in kSERCA results in a proportionately smaller decrease in SR calcium content (Figure 2). This relationship can be accounted for by the fact that calcium efflux from the SR is a steep function of SR calcium content. A decrease in kSERCA will decrease SR calcium content and even a small reduction of SR calcium will lower calcium release to balance uptake.