Prostaglandin endoperoxides (PGG2/PGH2), precursors of thromboxane (TX) A2 and prostaglandins, may accumulate sufficiently in the presence of a TXA2 synthase inhibitor to exert biological activity. To address whether this modulates the response to TXA2 synthase inhibition in the setting of thrombosis in vivo, we examined the interaction of a TXA2 synthase inhibitor (U63,557a) and a TXA2/prostaglandin endoperoxide receptor antagonist (L636,499) in a canine model of coronary thrombosis after electrically induced endothelial injury. U63,557a exerted little inhibitory effect in this model despite a marked reduction in serum TXB2 and urinary 2,3-dinor-TXB2, an index of TXA2 biosynthesis. Combination of the two drugs was more effective than either drug alone. The enhanced effect achieved upon addition of the TXA2/prostaglandin endoperoxide receptor antagonist to the TXA2 synthase inhibitor suggests that the response to the latter compound was limited by the proaggregatory effects of prostaglandin endoperoxides. The increased effect of the combination over the receptor antagonist alone may reflect metabolism of PGG2/PGH2 to platelet inhibitory prostaglandins. This is supported by the following findings: (a) urinary 2,3-dinor-6-keto-PGF1 alpha, an index of prostacyclin biosynthesis, increased after administration of the synthase inhibitor, an effect that was exaggerated in the presence of thrombosis; (b) inhibition of arachidonate-induced platelet aggregation by U63,557a was dependent on the formation of a platelet-inhibitory prostaglandin; and (c) pretreatment with aspirin abolished the synergism between these compounds. These studies demonstrate that prostaglandin endoperoxides modulate the response to TXA2 synthase inhibition in vivo and identify a drug combination of potential therapeutic efficacy in the prevention of thrombosis.