Platelets enzymatically convert prostaglandin H(3) (PGH(3)) into thromboxane A(3). Both PGH(2) and thromboxane A(2) aggregate human platelet-rich plasma. In contrast, PGH(3) and thromboxane A(3) do not. PGH(3) and thromboxane A(3) increase platelet cyclic AMP in platelet-rich plasma and thereby: (i) inhibit aggregation by other agonists, (ii) block the ADP-induced release reaction, and (iii) suppress platelet phospholipase-A(2) activity or events leading to its activation. PGI(3) (Delta(17)-prostacyclin; synthesized from PGH(3) by blood vessel enzyme) and PGI(2) (prostacyclin) exert similar effects. Both compounds are potent coronary relaxants that also inhibit aggregation in human platelet-rich plasma and increase platelet adenylate cyclase activity. Radioactive eicosapentaenoate and arachidonate are readily and comparably acylated into platelet phospholipids. In addition, stimulation of prelabeled platelets with thrombin releases comparable amounts of eicosapentaenoate and arachidonate, respectively. Although eicosapentaenoic acid is a relatively poor substrate for platelet cyclooxygenase, it appears to have a high binding affinity and thereby inhibits arachidonic acid conversion by platelet cyclooxygenase and lipoxygenase. It is therefore possible that the triene prostaglandins are potential antithrombotic agents because their precursor fatty acids, as well as their transformation products, PGH(3), thromboxane A(3), and PGI(3), are capable of interfering with aggregation of platelets in platelet-rich plasma.