Prostaglandins (PG) are synthesized by two isoforms of the enzyme PG G/H synthase [cyclooxygenase (COX)]. To examine selectivity of tolerated doses of an inhibitor of the inducible COX-2 in humans, we examined the effects of celecoxib on indices of COX-1-dependent platelet thromboxane (Tx) A₂ and on systemic biosynthesis of prostacyclin in vivo. Volunteers received doses of 100, 400, or 800 mg of celecoxib or 800 mg of a nonselective inhibitor, ibuprofen. Ibuprofen, but not celecoxib, significantly inhibited TxA₂-dependent aggregation, induced ex vivo by arachidonic acid (83 ± 11% vs. 11.9 ± 2.2%; P < 0.005) and by collagen. Neither agent altered aggregation induced by thromboxane mimetic, U46619. Ibuprofen reduced serum TxB₂ (−95 ± 2% vs. −6.9 ± 4.2%; P < 0.001) and urinary excretion of the major Tx metabolite, 11-dehydro TxB₂ (−70 ± 9.9% vs. −20.3 ± 5.3%; P < 0.05) when compared with placebo. Despite a failure to suppress TxA₂-dependant platelet aggregation, celecoxib had a modest but significant inhibitory effect on serum TxB₂ 4 hr after dosing. By contrast, both ibuprofen and celecoxib suppressed a biochemical index of COX-2 activity (endotoxin induced PGE₂ in whole blood ex vivo) to a comparable degree (−93.3 ± 2% vs. −83 ± 6.1%). There was no significant difference between the doses of celecoxib on COX-2 inhibition. Celecoxib and ibuprofen suppressed urinary excretion of the prostacyclin metabolite 2,3 dinor 6-keto PGF_1α. These data suggest that (i) platelet COX-1-dependent aggregation is not inhibited by up to 800 mg of celecoxib; (ii) comparable COX-2 inhibition is attained by celecoxib (100–800 mg) and ibuprofen (800 mg) after acute dosing; and (iii) COX-2 is a major source of systemic prostacyclin biosynthesis in healthy humans.