The vascular tension in the coronary artery is modulated by factors released by endothelial cells. We investigated the relationship between the Ca²⁺ level in endothelium and endothelium-mediated changes in smooth muscle tone in high K⁺-depolarized canine coronary arteries by measuring intracellular Ca²⁺ concentration fluorimetrically with the Ca²⁺ indicator fura 2. Addition of Ca²⁺ (1 mM) caused an increase in endothelial Ca²⁺ and relaxed the 30 mM K⁺-depolarized arteries following inhibition of Ca²⁺ influx in the smooth muscle with diltiazem. This relaxation was inhibited by N^G-monomethyl-L-arginine. As extracellular K⁺ concentration was decreased, increases of endothelial Ca²⁺ were augmented, whereas the relaxation was decreased. Basal muscle tone was found to be decreased in low K⁺ by measuring relaxation by sodium nitroprusside. These results suggest the importance of Ca²⁺ level in the endothelium in playing a modulatory role in coronary tension through the production of nitric oxide. The correlation of extracellular K⁺ to Ca²⁺ level in the endothelium indicates a typical characteristic of the passive Ca²⁺ entry pathway in the endothelium, whereas the resultant relaxation appears to be restricted by the basal muscle tone.