The effects of vanadate on cellular Ca²⁺ movements across the sarcolemma of cardiac muscle cells were investigated by measuring the intracellular and extracellular Ca²⁺ activities of guinea pig papillary muscle with Ca²⁺-selective electrodes. During the rest period following a steady-state of 2 contractions per second the extracellular Ca²⁺ concentration was increased over the basal level within a minute. During the rest period Ca²⁺ was transported across the sarcolemma into the extracellular space. Vanadate decreased the change in extracellular Ca²⁺ concentration during the rest period implying that the Ca²⁺ efflux across the sarcolemma was decreased by vanadate. Vanadate increased intracellular Ca²⁺ activities significantly (from 1.9 × 10^-7 M to 10^-6 M) resulting in an increase in resting tension. These results suggest that vanadate decreases Ca²⁺ efflux from the cells into the extracellular space by blocking Ca²⁺ transport across the sarcolemma, possibly blocking the Na⁺ - Ca²⁺ exchange transport.