Changes of the activity of the sarco-endoplasmic reticulum Ca 2+-ATPase (SERCA) affect the amplitude of the systolic Ca 2+ transient and thence cardiac contractility. This is thought to be due to alterations of SR Ca 2+ content. Recent work on mice in which the expression of SERCA is decreased found that a large reduction of SERCA expression resulted in a proportionately much smaller decrease of SR Ca 2+ content. The aim of the current work was to investigate the quantitative nature of the dependence of both the amplitude of the systolic Ca 2+ transient and SR Ca 2+ content on SERCA activity during acute partial inhibition of SERCA. Experiments were performed on rat ventricular myocytes. Brief application of thapsigargin (1 μm) resulted in a decrease of SERCA activity as measured from the rate of decay of the systolic Ca 2+ transient. This was accompanied by a decrease in the amplitude of the systolic Ca 2+ transient which was linearly related to that of SERCA activity. However, the fractional decrease in the SR Ca 2+ content was much less than that of SERCA activity. On average SR Ca 2+ content was proportional to SERCA activity raised to the 0.38 ± 0.07 power. This shallow dependence of SR content on SERCA activity arises because Ca 2+ release is a steep function of SR Ca 2+ content. In contrast SR Ca 2+ content was increased 4.59 ± 0.40 (n= 8)-fold by decreasing ryanodine receptor opening with tetracaine (1 mm). Therefore a modest decrease of SR Ca 2+ content results in a proportionately larger fall of Ca 2+ release from the SR which can balance a larger initiating decrease of SERCA. In conclusion, the shallow dependence of SR Ca 2+ content on SERCA activity is expected for a system in which small changes of SR Ca 2+ content produce larger effects on the amplitude of the systolic Ca 2+ transient. © 2011 The Authors. Journal compilation © 2011 The Physiological Society.