The changes in extracellular calcium activities during contractures of guinea pig atrial trabecular muscles were measured with Ca²⁺-selective electrodes. When the tissue was superfused with Na⁺-free Tyrode solution extracellular Ca²⁺ activities were decreased and contractures were induced with some delay. When the contracture was relaxed with Na⁺-containing Tyrode solution, extracellular Ca²⁺ activities were increased transiently and recovered in a Na⁺-dependent manner. The magnitude of extracellular Ca²⁺ activity decreased was proportional to the maximum magnitude of contracture induced by Na⁺-free solution. Addition of caffeine (10 mM) to Na⁺-free solution induced transient contracture following slow development of contracture and an increase in extracellular Ca²⁺ activity. Removal of caffeine from Na⁺-free solution caused a slow relaxation of contracture and a decrease in extracellular Ca²⁺ activity. These results confirm that caffeine blocks Ca²⁺ uptake by the sarcoplasmic reticulum (SR) resulting in an increase in sarcoplasmic Ca²⁺ activity. Ca²⁺ activity in the extracellular space, the amount of Ca²⁺ transported into the cell(Ca²⁺ depletion in the extracellular space), and the magnitude of contracture are well correlated. Present experiments suggest that extracellular use of Ca²⁺-selective electrodes provides continuous and quantitative monitoring of Na⁺-dependent Ca²⁺ flux across the cardiac cell membrane.