Effects of phorbol ester, 12-deoxyphorbol 13-isobutyrate (DPB), on muscle tension and cytosolic Ca²⁺ ([Ca²⁺]_i) level was investigated in rat anococcygeus muscle in comparison with other smooth muscles. 1) DPB (10^-6 M) induced a large contraction and an elevation of [Ca²⁺]_i level in rat aorta and small and rhythmic changes in tension and [Ca²⁺]_i level in guinea pig ileum. However, DPB did not change either of the parameters in rat anococcygeus muscle. 2) DPB caused tension development without changing the [Ca²⁺]_i level elevated by high K⁺, ionomycin or β-escin in the anococcygeus muscle. 3) In the β-escin permeabilized muscles of guinea pig ileum and urinary bladder, rabbit mesenteric artery and rat anococcygeus muscle, DPB enhanced the Ca²⁺-developed tension. Moreover, the enhancement was inhibited by H-7 (3 x 10-5 M). 4) DPB did not cause muscle tension to develop in the muscle of rat aorta, guinea pig ileum and rat anococcygeus muscle, pretreated with phorbol 12-myristate 13-acetate for 24 hr. In conclusion, DPB showed different contractile effects on the aorta, ileum and anococcygeus muscle, respectively. The initiation of muscle tension by DPB probably requires [Ca²⁺]_i and the DPB-induced enhancement may be due to a Ca²⁺ sensitization of contractile elements in the anococcygeus muscle. Therefore, the difference between the DPB-induced response of the anococcygeus muscle and those of the other muscles seems to be due to a different Ca²⁺ movement caused by DPB. Moreover, it is suggested that DPB develops muscle tension by increasing [Ca²⁺]_i and enhances it through the mediation of protein kinase C in the anococcygeus muscle as well as the other smooth muscles.