To compare the inhibitory effects of aspirin on prostaglandin synthesized by vessel walls and platelets, we obtained vein segments from five subjects before they were given 150 or 300 mg of aspirin and at various intervals afterward. We then measured prostacyclin (PGI2) synthesis with a radioimmunoassay for its stable metabolite, 6-keto-prostaglandin F1 alpha. Platelet production of thromboxane A2 was measured with a radioimmunoassay for its stable metabolite, thromboxane B2. Two hours after aspirin had been given, 81 to 100 per cent inhibition of PGI2 synthesis was demonstrated; 86 per cent inhibition was still evident in one subject tested eight hours after administration. Simultaneously, platelet production of thromboxane B2 was completely inhibited for more than 24 hours. We conclude that there is little difference between the initial inhibitory response of platelet cyclooxygenase and that of vessel-wall cyclooxygenase to these doses of aspirin. Our results also indicate that in male subjects the prolonged template bleeding time after aspirin is not the consequence of selective inhibition of platelet production of thromboxane.