The purpose of this study was to compare the effect of aspirin on the release of metabolites of arachidonic acid from thrombin and collagen stimulated platelets. Human platelets were incubated with tritium-labeled arachidonic acid and then isolated by gel filtration. The labeled platelets were stimulated with varied doses of either thrombin or collagen for 15 minutes. The platelets were then pelleted and the released metabolites of arachidonic acid were separated by high-performance liquid chromatography. In experiments with aspirin, the aspirin was added 5 minutes before either thrombin or collagen. The total release of radioactivity was comparable at 15 μg/ml of collagen and 1.0 units/ml of thrombin (approximately 10% of the total) and at 100 μg/ml of collagen and 5 units/ml of thrombin (approximately 30%). Aspirin (25 μg/ml) preferentially inhibited collagen-stimulated release of radioactivity (62% inhibition of release with 15 μg/ml of collagen vs. 25% inhibition of release with 1.0 units/ml of thrombin; 54% inhibition of release with 100 μg/ml of collagen vs. 8% inhibition of release with 5.0 units/ml of thrombin). At all concentrations of collagen or thrombin, cyclo-oxygenase activity was markedly reduced by aspirin. The selective effect of aspirin on collagen reflects primarily preferential suppression of HETE formation. We conclude that aspirin inhibits the formation of both lipoxygenase and cyclooxygenase-derived products in collagen-stimulated platelets.