—The aim of this study was to investigate how sarcoplasmic reticulum (SR) Ca²⁺ content and systolic Ca²⁺ are controlled when Ca²⁺ entry into the cell is varied. Experiments were performed on voltage-clamped rat and ferret ventricular myocytes loaded with fluo-3 to measure intracellular Ca²⁺ concentration ([Ca²⁺]_i). Increasing external Ca²⁺ concentration ([Ca²⁺]_o) from 1 to 2 mmol/L increased the amplitude of the systolic Ca²⁺ transient with no effect on SR Ca²⁺ content. This constancy of SR content is shown to result because the larger Ca²⁺ transient activates a larger Ca²⁺ efflux from the cell that balances the increased influx. Decreasing [Ca²⁺]_o to 0.2 mmol/L decreased systolic Ca²⁺ but produced a small increase of SR Ca²⁺ content. This increase of SR Ca²⁺ content is due to a decreased release of Ca²⁺ from the SR resulting in decreased loss of Ca²⁺ from the cell. An increase of [Ca²⁺]_o has two effects: (1) increasing the fraction of SR Ca²⁺ content, which is released on depolarization and (2) increasing Ca²⁺ entry into the cell. The results of this study show that the combination of these effects results in rapid changes in the amplitude of the systolic Ca²⁺ transient. In support of this, the changes of amplitude of the transient occur more quickly following changes of [Ca²⁺]_o than following refilling of the SR after depletion with caffeine. We conclude that the coordinated control of increased Ca²⁺ entry and greater fractional release of Ca²⁺ is an important factor in regulating excitation-contraction coupling.