Biphasic contractions were produced in dog trabeculae by replacing 90-95% of the calcium in the bathing solution with strontium. These conditions produced prolonged action potentials accompanied by contractions with two distinct phasic components. The early component disappeared slowly when the remaining Ca++ was removed, whereas the late component was eliminated quickly when Sr++ was removed. Manganese ion (0.25 mM) preferentially decreased the late component without changing the action potential, whereas caffeine and ryanodine decreased or eliminated the early component. Ryanodine did not alter the action potential. Isoproterenol rapidly increased the early component and, more slowly and to a lesser degree, increased the late component. The results suggest that the early component is caused by intracellular release of activator cation, probably from the sarcoplasmic reticulum, whereas the late component is the result of Sr++ entry across the sarcolemma, possibly by way of the slow inward current.