Effects of 3 Ca2+ antagonists, diltiazem, nifedipine and verapamil, on the neuromuscular junction were investigated in the in situ canine anterior tibial muscle preparation by close-arterial injections via the femoral artery. The Ca2+ antagonists dose-dependently augmented the twitch tension elicited by indirect stimulation applied to the common fibular nerve at a frequency of 0.1 or 0.2 Hz. The maximum of the twitch tension reached .apprx. 130% of the control by 10 mg of nifedipine, or 30 mg of diltiazem or verapamil. Nifedipine was .apprx. 3 times as potent as diltiazem or verapamil in this augmentation. The doses of Ca2+ antagonists had little effects on the twitch tension elicited by direct stimulation of the muscle. The tetanic contraction elicited by indirect stimulation of 30 Hz was partially inhibited and the post-tetanic potentiation of the twitch tension was slightly depressed with diltiazem and verapamil. Nifedipine induced slight increases in the tetanic contraction and the post-tetanic potentiation. Ca2+ antagonists depressed the acetylcholine-induced contraction of the skeletal muscle.