Platelet membrane vesicles accumulated Ca 2+ in an ATP-dependent fashion, and 25–50% of the accumulated Ca 2+ was released by the addition of 10 μM inositol 1,4,5-trisphosphate (IP 3). The concentration of IP 3 required for half-maximal Ca 2+ release was approximately 0.5 μm. The inhibition of IP 3-induced Ca 2+ release from these membrane vesicles by various agents was examined. Of the plasma membrane Ca 2+ channel blockers, cinnarizine and flunarizine were found to be potent inhibitors of IP 3-induced Ca 2+ release while having no effect on ATP-dependent Ca 2+ uptake. The ic 50 value for both cinnarizine and flunarizine as inhibitors of IP 3-induced Ca 2+ release was below 10 −6M. Nifedipine, verapamil, bepridil, and diltiazem did not significantly inhibit IP 3-induced Ca 2+ release at the highest concentration tested (50 μM). The “intracellular Ca 2+ antagonists” ryanodine, TMB-8 (8-( N, N-diethylamino)octyl-3,4,5-trimethoxybenzo dantroline, trifluoperazine and chlorpromazine were not inhibitors of IP 3-induced Ca 2+ release at 50 μM. The local anesthetics benzocaine and lidocaine weakly inhibited the IP 3-induced Ca 2+ release with ic 50 values of approximately 5 and 50 μM, respectively, whereas other local anesthetics tested were less potent inhibitors. The potent inhibitors described may prove useful as probes of the IP 3-induced Ca 2+ release channels.