Cardiac membrane vesicles take up Ca2+ in response to Na+ gradient (high inside) and negative inside membrane potential. Both components of the Ca2+ uptake, the Na+ gradient dependent uptake and the membrane potential dependent uptake are inhibited by verapamil; the action is dose-dependent and the concentrations of verapamil required to inhibit the Ca2+ uptake to 50% of its maximal value are 50 and 60 microM respectively. In the concentration ranges tested (50-750 microM Ca2+), the inhibitory effect of verapamil could not be antagonized by increasing the Ca2+ concentration of the medium. Introducing verapamil into the vesicles by rapid freezing and slow thawing of the vesicles had the same inhibitory effect as adding the same concentration of verapamil on the outside of the vesicles. Adding verapamil to both sides of the vesicle membrane led to higher inhibition of Ca2+ uptake. It is proposed that addition of verapamil can cause a change in cardiac membranes which is manifested by a decrease in the driving membrane potential and Ca2+ transport.