The effect of Ca antagonist on the contractile apparatus was investigated in glycerinated cardiac muscle preparations obtained from canine hearts. Each muscle preparation had three consecutive isometric contractions. The 1st and 3rd contractions were produced with a control contraction solution, and compared with the 2nd contraction which was induced with a contraction solution containing verapamil. The results showed that maximal developed tension (Po) was enhanced significantly by 1.02 X 10(-2) mM of verapamil, and the augmentation of contractility was dependent on the concentrations of verapamil. Thus, not only Po, but also dT/dt increased tremendously at 1.02 mM of verapamil. Such contractile potentiation by verapamil was also ascertained by another Ca antagonist, Diltiazem hydrochloride. The developed tension was maximum at pCa 4.0, and no developed tension was found at pCa 8.0. The relationship between pCa and tension with verapamil shifted to the left from that without verapamil, showing higher sensitivity to Ca2+. From these results, it was strongly indicated that Ca antagonist is a potentiating agent of the contractile force.