In circular muscles of the canine stomach, diltiazem (greater than 3 X 10(-6)M) depolarized and nicardipine (greater than 10(-6)M) hyperpolarized the membrane. Diltiazem (greater than 5 X 10(-6)M) or nicardipine (greater than 3 X 10(-7M) inhibited the plateau potential of spontaneously generated or electrically evoked action potential. In Na-deficient solution (between 137 and 30 mM Na), diltiazem and nicardipine selectively inhibited the plateau potential. In Na- (15 mM Na) or Ca-deficient (0.25 mM Ca) solution, spontaneously generated action potential ceased, and only graded responses were evoked by electrical stimulation. These graded responses were inhibited by diltiazem (greater than 5 X 10(-6)M) or nicardipine (greater than 3 X 10(-7)M). The ionic contribution for generation of action potential in this muscle cell was discussed in relation to actions of Ca antagonists.