The effect of cilostazol, a phosphodiesterase III inhibitor, on catecholamine secretion was examined using bovine adrenal chromaffin cells in culture. Catecholamine secretion evoked by acetylcholine was markedly inhibited by cilostazol. In contrast, 56 mM K+-evoked secretion was slightly inhibited by cilostazol. Cilostazol elevated the level of cyclic AMP in the cells. However, a cyclic AMP analog (dibutyryl cAMP) or an adenylate cyclase activator (forskolin) failed to inhibit secretion of catecholamine induced by acetylcholine. Cilostazol decreased the level of intracellular free Ca2+ stimulated by acetylcholine. Cilostazol thus inhibits secretion of catecholamine through its blocking action on Ca2+ movement in the adrenal chromaffin cells.