Prostaglandins stimulate cyclic AMP synthesis in platelets, leading to inhibition of platelet aggregation. Prostaglandin receptors have been identified but knowledge of their pharmacology is limited by the lack of antagonists. Kinetic measurements of prostaglandinregulated cyclic AMP formation provide useful information on prostaglandin interactions. A model is presented in which prostaglandins interact with adenylate cyclase through separate stimulatory and inhibitory receptors that differ in their affinity for prostaglandins. The model explains differences in cyclic AMP metabolism between different prostaglandins and provides a mechanism for desensitisation.