The influence of a diet supplemented with MaxEPA (a fish oil concentrate, rich in eicosapentaenoic acid = EPA) on bleeding time in small mesenteric arteries of the rat and on formation of thromboxane B2 (TXB2) by ADP-stimulated platelets was investigated. Since EPA has been found to antagonize noradrenaline (NA) - induced vasoconstriction, the influences on bleeding time of the alpha-receptor antagonist phentolamine and of the adrenergic neuron blocking agent guanethidine alone as well as during dietary supplementation with MaxEPA were studied. Bleeding time slowly increased during the MaxEPA diet over a period of 12 weeks resulting in a final prolongation of about 75%. Indomethacin which dose-dependently increased bleeding time in control rats, further increased the already prolonged bleeding time in rats on the MaxEPA diet. Guanethidine prolonged bleeding time strongly in control animals but caused no further increase in those on the MaxEPA diet. Indomethacin also further increased the bleeding time prolonged by guanethidine, just as it did in animals on MaxEPA diet. In contrast to guanethidine, phentolamine prolonged bleeding time only slightly. The ability of platelets from rats on the diet to generate TXB2 was reduced to 59.5% but was more drastically reduced (to 26.7%) from rats pretreated with indomethacin. The results indicate that MaxEPA feeding prolongs bleeding time by a mechanism other than that of indomethacin. We speculate that EPA prolongs bleeding time rather by antagonizing the vasoconstrictor effect of sympathetic transmitters at the site of the injured vessel than by a reduced formation of TXA2.