Platelet aggregation was studied in vitro with human platelets and in vivo in guinea pigs and rabbits. L-640035 and its acetic acid metabolite, L-636499, significantly inhibited human platelet aggregation induced by arachidonic acid, collagen and a prostaglandin endoperoxide analog (U44069) but not ADP. In guinea pigs, circulating platelets labeled with 111I were monitored with probes in the lung and abdominal regions. Platelet aggregation was indicated by an increase in the ratio of .gamma.-radiation of the lung vs. the abdominal region, as aggregates of platelets accumulate in the microvascular network in the lung. L-640035 (1 and 3 mg/kg i.v.) inhibited platelet aggregation induced in this model by U-44069 but not by ADP. In rabbits acute thrombosis was induced in the carotid artery by local electrical stimulation and platelet accumulation at the stimulus site was quantitated using 111I-labeled autologous platelets. L-640035 (1 and 3 mg/kg i.v.) significantly reduced electrically evoked platelet accumulation. L-640035 is evidently a novel and selective antagonist of platelet aggregation induced by thromboxane A2 or prostaglandin endoperoxides and may have utility as an antiplatelet drug.