Effects of high concentration of KCI and caffeine on cytosolic Ca²⁺ level ([Ca²⁺]_cyt), measured simultaneously with muscle tension using a fluorescent intracellular Ca²⁺ indicator fura 2, were examined in isolated smooth muscle of rat aorta. High K⁺ (72.7 mM) solution induced sustained increase in both ([Ca²⁺]_cyt) and tension. In contrast to this, caffeine (20 mM) induced a rapid increase in ([Ca²⁺]_cyt) followed by a decrease to a level which was higher than the resting level. However, muscle tension showed only a transient increase followed by a decrease below the resting level. In a Ca²⁺-free solution, high K⁺-induced neither ([Ca²⁺]_cyt) nor tension, whereas caffeine induced a transient increase in both ([Ca²⁺]_cyt) and muscle tension. These results suggest that high K⁺-induced contraction in vascular smooth muscle of rat aorta is due to Ca²⁺ influx whereas caffeine-induced contraction is due to Ca²⁺ release from cellular store. Further, caffeine seems to have an additional effect to decrease the sensitivity of the contractile elements to Ca²⁺.