Lipoxygenase products, which are formed in great amounts in platelets during their activation, have been prepared from arachidonic acid (20:4n-6), the main polyunsaturated fatty acid (PUFA) esterified in platelet phospholipids, and from two major PUFAs of fish fat, eicosapentaenoic (20:5n-3) and docosahexaenoic (22:6n-3) acids. These compounds have been synthesized using platelet suspension as enzymic source, purified by high performance liquid chromatography, and their structure were checked by gas chromatography-mass spectrometry. Their effects were investigated in vitro upon human platelet aggregation induced by 11,9-epoxy-methano-analogue of PGH2 (U-46619) and upon thromboxane A2-induced vasoconstriction of rabbit aorta. All hydroxylated fatty acids inhibited U-46619-induced aggregation in a concentration-dependent fashion. Compounds issued from 22:6n-3 were the most potent inhibitors and their IC50 differed significantly from that of 12-hydroxy-eicosatetraenoic acid (12-HETE). Among them, 14-hydroxy-docosahexaenoic acid (14-OH-22:6) was the most effective anti-aggregating molecule (IC50:0.45 microM). 10 microM 12-HETE and 14-OH-22:6 inhibited 60% and 75% of smooth muscle contraction induced by TXA2-like material, respectively. At 1 microM, solely 14-OH-22:6 had an inhibitory effect on adrenaline-, angiotensine- or histamine-induced contraction. Since thromboxane receptors in platelets and vascular smooth muscle cells present strong similarities, it is concluded that hydroxylated fatty acids can antagonize prostanoid action probably by interfering with their receptor sites.