Flunarizine and cinnarizine (IC50 6.8x10(-6) and 2.8x10(-5) M, respectively) inhibited 3H-serotonin uptake by platelets. In higher doses, they blocked serotonin-induced platelet aggregation and stimulated 3H-serotonin release from these cells. Imipramine did not affect serotonin-releasing effects of preparations. In all patients cinnarizine was more potent in inhibiting serotonin uptake, and in half of the patients cinnarizine displayed higher activity as an inductor of serotonin release.