The in vitro effects of aspirin and other non steroidal antiinflammatory drugs (NSAIDs) on 12-HETE formation were evaluated in platelet rich plasma (PRP) stimulated with collagen. Aspirin (1 X 10(-4)-1 X 10(-3) M) inhibited 12-HETE production in PRP. Inhibition of 12-HETE formation was detected also in PRP incubated with indomethacin and BW 755C. Sodium salicylate, instead, did not reduce 12-HETE synthesis. Aspirin (3 X 10(-3) M) did not modify 12-HETE synthesis in similarly challenged washed platelet (WP) preparations, inspite of its effect on the cyclooxygenase pathway. It may be concluded that aspirin and other NSAIDs, not only affect the cyclooxygenase pathway, but also inhibit 12-HETE synthesis. In addition it is shown that the 12-HETE synthesis is reduced by plasma and that this effect is enhanced in the presence of aspirin.