Abstract We studied the effects of caffeine on inositol 1,4,5-trisphosphate (IP 3)-induced Ca 2+ release in saponin-permeabilized smooth muscle cells. The Ca 2+ concentration in the vicinity of the Ca 2+ store was strongly buffered to exclude the secondary effect of Ca 2+ on the Ca 2+ release. 2 mM caffeine shifted the IP 3 concentration-Ca 2+ release relation at 100 nM Ca 2+ to the higher IP 3 concentration without decreasing the steepness of the relation. The ascending limb of the biphasic Ca 2+ dependence of IP 3-induced Ca 2+ release was shifted to the higher Ca 2+ concentration by 2 mM caffeine. However, caffeine showed little inhibitory effect in the absence of Ca 2+. These results indicate that caffeine inhibits IP 3-induced Ca 2+ release only when the Ca 2+ release is potentiated by Ca 2+.