For 2 mo rats were fed a salmon oil diet (12.5%, wt/wt) supplemented with 4.5% (wt/wt) corn oil, a corn oil diet (17%, wt/wt) or a low fat diet (4.4%, wt/wt). Cardiac lipids were analyzed and fatty acid composition of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) was determined. Ventricular biopsies were taken for ultramicroscopic examination. Serum cholesterol, triglyceride, phospholipid and vitamin E concentrations were significantly lower in rats fed salmon oil than in those fed the other two diets, whereas serum transaminases and vitamin A were not significantly affected. Cardiac protein, phospholipid, triglyceride and cholesterol concentrations were unaffected by diet. Cardiac phospholipid composition remained unchanged and no significant changes in lyso-PC or lyso-PE levels were observed. However, the salmon oil diet produced a markedly lower n-6/n-3 ratio in both PE and PC than in the other two diets. This was the result of replacement of n-6 polyunsaturated fatty acids (PUFA), primarily 20:4n-6 with n-3 PUFA, primarily 22:6n-3. The unsaturation index of PC and PE was higher with the salmon oil diet than with the other two diets. Ventricular biopsies of rats fed salmon oil showed mild lipid accumulation associated with some lipofuscin-like material. It is suggested that, in rat heart, fish oil led to a moderate accumulation of lipids, the composition of which may include long-chain monounsaturated fatty acids and a degradative form of peroxidized lipids.