We investigated the influence of fish oil on cholesterol induced atherosclerosis in rabbits. Group I, a control group was fed a cholesterol-free diet, group II was fed a diet supplemented with 1.5% cholesterol, group III received in addition to cholesterol supplementation a purified fish oil concentrate (Maxepa, 2 ml/d). The animals received these diets for 5 months (100 g/d). Aortic atherosclerosis as measured by planimetry of sudanophilic lesions was significantly higher (+59%) in group III as compared with group II, even though serum cholesterol levels were comparable. No differences were found in platelet half-life times between groups II and III, but these values were significantly lower than the half-life of platelets in the control group I. Total serum peroxide levels, expressed as malondialdehyde equivalents were significantly elevated in the fish oil-treated group. This may be due to malondialdehyde modification of the lipoproteins and may be responsible for the enhanced development of atherosclerosis in these animals.