Prostacyclin (PGX) (57)-9-deoxy-6,9alpha-epoxy-delta5-PGF1alpha has been found to be a potent stimulator of cAMP accumulation in platelets than PGE1. The prostacyclin stimulation of platelet cAMP accumulation can be antagonized by the prostaglandin endoperoxide PGH2, and a PGH2-induced platelet aggregation is antagonized by prostacyclin. A model of platelet homeostasis is proposed that suggests platelet aggregation is controlled by a balance between the adenylate cyclase stimulating activity of prostacyclin, and the cAMP lowering activity of PGH2.