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1Caroverine fumarate, 1‐(2‐diethylaminoethyl)‐3‐(p‐methoxy‐benzyl)‐1,2‐dihydro‐2‐quinoxalinone fumarate, caused a greater inhibition of the pressor response to KCl (8 × 10−2m) than that to noradrenaline (10−6m) in the rat hindquarter preparation.2In the isolated aorta of the rat, caroverine (up to 10−6m) markedly suppressed the contraction caused by KCl (4 × 10−2m) (high‐K) but had little effect on the contractile response to noradrenaline (10−6m) whether added before the spasmogen or in its presence.3In the high‐K‐treated aorta, caroverine shifted the concentration‐response curve for external calcium to the right, competitively. The negative logarithm of the affinity (pA2) of caroverine was calculated to be approx. 7.4Increased 45Ca uptake of the high‐K‐treated aorta measured by a modified lanthanum method was inhibited by either caroverine (3 × 10−6m) or verapamil (10−6m).5Concentrations of caroverine and verapamil reducing high‐K‐induced aortic contraction to 50% of its maximum were 2.4 × 10−7and 6.6 × 10−8mrespectively.6Following washout the caroverine‐induced inhibition of high‐K‐induced aortic contraction was more rapidly restored than the verapamil‐induced inhibition.7These results suggest that caroverine fumarate is a specific and readily reversible calcium influx inhibitor in the rat vascular smooth muscle.