Aqueous extracts of guaraná were studied in terms of effects on the aggregation of human and rabbit platelets. Guaraná extracts have anti-aggregatory and de-aggregatory actions on platelet aggregation induced by ADP or arachidonate but not by collagen. The active material was shown to be water soluble and heat resistant and appeared to be different from salicylates, nicotinic acid or known xanthines. Guaraná extracts inhibited platelet aggregation in rabbits following either intravenous or oral administration.