The effects of single oral administrations of acetylsalicylic acid (ASA, 500 mg), indomethacin (Indo, 50 mg) and sodium salicylate (NaSal, 400 mg) on platelet aggregation and on the thromboxane B2 (TXB2) and 12-hydroxyeicosatetraenoic acid (12-HETE) synthesis by platelet rich plasma (PRP) stimulated with collagen were evaluated. While both ASA and Indo significantly inhibited TXB2 synthesis and platelet aggregation, significant reduction of 12-HETE formation at 2 and 6 h after the administration of the drug, was detected only in subjects who ingested ASA. NaSal did not affect any of the tested parameters. The comparison of the effect of ASA (200 mg) on 12-HETE synthesis in washed platelets and PRP shows that the drug is able to affect this parameter only in PRP. To obtain a constant inhibition of 12-HETE synthesis in PRP over a 24 h period, a repeated ASA treatment schedule was assessed (ASA 200 mg every 6 h for 5 times). TXB2 synthesis in PRP was almost completely suppressed at 2 h after the first ASA administration and inhibition remained constant up to 48 h after the last ASA intake. As far as 12-HETE synthesis by stimulated PRP is concerned, a significant reduction of this parameter was detected at 4 h after the first drug administration and the levels remained almost constant following the repeated administrations during a 24 h period. These data indicate that ASA, but not Indo and NaSal, significantly affect not only TXB2 synthesis but also 12-HETE formation in PRP. The lack of the effect of ASA administration on 12-HETE, found when studies were carried out in washed platelets, indicates that the drug requires the presence of plasma factors for its activity on the formation of 12-lipoxygenase products by platelets.