Relaxing effects of nitroprusside and verapamil on the resting or K-stimulated tension and 45Ca uptake in human umbilical vessels were investigated. Both nitroprusside and verapamil reduced the resting and K-stimulated tensions in a dose-dependent manner. Nitroprusside was much more effective than verapamil in reducing resting tension. However, verapamil caused a greater relaxation of the K-induced contraction than nitroprusside. In a Ca2+-free medium, K+ still caused a small and sustained contraction. Nitroprusside abolished the contraction in the Ca2+-free medium, which verapamil failed to inhibit. Neither nitroprusside nor verapamil affected the resting 45Ca uptake measured by a lanthanum method. The K-induced increase in 45Ca uptake was inhibited by verapamil, but not by nitroprusside. The results suggest that nitroprusside unlike verapamil does not act as a Ca2+ influx inhibitor resulting in the relaxation of human umbilical vessels.