The influence of captopril on blood platelet aggregation and on serotonergic mechanisms in blood platelets were studied. In rats pretreated with captopril (10.0 mg/kg p.o.) platelet aggregation induced by ADP and collagen was significantly reduced. In vitro this drug had no inhibitory effect. Besides, captopril did not change the amplifying effect of serotonin on platelet aggregation. Captopril also had no influence on the uptake and storage of 5-hydroxytryptamine by rat blood platelets. These results show that serotonergic mechanisms are not involved in the suppressive effect of captopril on platelet aggregation.