To elucidate the Ca²⁺ release mechanisms in the rabbit coronary artery, arterial preparations were permeabilized with β-escin and changes in tension were measured under varying experimental conditions. Additionally, we investigated properties and distribution of two kinds of Ca²⁺ release mechanisms, Ca²⁺-induced Ca²⁺ release (CICR) and IP₃-induced Ca²⁺ release (IICR). The results obtained were summarized as follows; 1. When a rabbit coronary artery was incubated in a relaxing solution containing 30 µM β-escin for 40 min. sensitivity to externally added Ca²⁺ was much higher in β-escin permeabilized muscle than in intact preparations. The contractile effect of IP₃ in β-escin permeabilized muscle was also demonstrated; 2. Caffeine and IP₃ contracted coronary arteries were permeabilized with β-escin, but the amplitude of contraction was much larger in the presence of caffeine than of IP₃. 3. Intracellular heparin completely inhibited the contractions induced by IP₃, but not those by caffeine. On the other hand, procaine inhibited the responses to caffeine, but not those to IP₃. Ryanodine inhibited both the caffeine- and IP₃-induced contractions. 4. The amplitude of contractile responses was much larger to the maximal stimulation of CICR by applying caffeine than to the maximal stimulation of IICR by applying IP₃. After the maximal CICR stimulation by caffeine, the activation of IICR by IP₃ without the reloading of Ca²⁺ could no longer evoke contraction. On the other hand, after the maximal IICR activation, the activation of CICR could still evoke contraction although the amplitude of the contraction was smaller when compared with the case without the initial IICR stimulation. 5. Acetylcholine contracted coronary artery smooth muscles were permeabilized with β-escin. However, in the absence of added guanosine triphosphate (GTP), the responses were very small. Acetylcholine-induced contraction was inhibited by heparin, but not by procaine. From the above results, it may be concluded that there are two kinds of mechanisms of Ca²⁺ release, CICR and IICR, in the rabbit coronary artery smooth muscle cell. Also, whereas the CICR mechanism distributes on the membrane of the whole smooth muscle Ca²⁺ store, the IICR mechanism distributes only on a part of it.