Quadrigemine B, quadrigemine A, isopsychotridine C and psychotridine, isolated from PSYCHOTRIA FORSTERIANA, are potent inhibitors of the aggregation of washed human platelets induced by ADP, collagen or thrombin. The four compounds are active in the 1-10 microM range. The quadrigemine-type alkaloids do not increase the level of platelet cyclic AMP, either alone or in the presence of 20 nM prostaglandin E, (PGE (1)), an activator of adenylate cyclase. The characteristics of the pharmacological action of these compounds suggest that they act at a later stage in platelet activation, possibly through an interaction with cytoskeletal proteins.