Verapamil (10(-6) M) significantly reduced a development of the membrane depolarization and the contracture which were induced by Na removal. The fully developed depolarization, after exposure to Na-free solution, was reduced greatly by verapamil, excess Ca (15 mM) or sodium nitroprusside (10(-6) M), but some depolarization still remained. This remaining depolarization was completely blocked by readmission of 10 mM Na to Na-free solution containing verapamil, excess Ca or sodium nitroprusside. However, Mn (1.5 mM) did not block the depolarization due to Na removal and if Mn was present in Na-free solution, readmission of 15 mM Na to Na-free solution was not able to block the depolarization. On the other hand, the contracture due to Na removal was inhibited by addition of Mn (1.5 mM) or sodium nitroprusside (10(-6) M), but partially by addition of verapamil (10(-6) M) or excess Ca, to Na-free solution. These remaining contractures were also abolished by readmission of 10 mM Na to Na-free solution containing verapamil or excess Ca. Thus, these results suggest that the depolarization and the contracture due to Na removal are induced by increase in cytoplasmic Ca concentrations through (1): verapamil-sensitive Ca channel, (2): Na-Ca exchange system and (3): some mechanism which is inhibited by sodium nitroprusside.