The investigation was attempted to clarify the effects of fish oil on the concentration of lipids in serum and lipoproteins in rats fed diets differing in cholesterol and fat. Male Sprague-Dawley rats were maintained on low-fat/high-fat diets without and with 1.5% cholesterol (base diets) for 28 days. Half of each group was then switched to a fish oil diet for 20 days with 5.6% fish oil for exchange of coconut oil and beef tallow. Total cholesterol in rat serum was increased following feeding high amounts of dietary cholesterol. This increase was due to raised VLDL and LDL cholesterol. Rats fed the high-cholesterol/high-fat diet had lower HDL cholesterol concentration than groups fed the other base diets. Dietary fish oil lowered serum and lipoprotein cholesterol, even in the presence of dietary cholesterol. In rats fed the high-fat/cholesterol-free diet triglyceride levels in total serum and VLDL were higher than in rats fed the other base diets. The hypertriglyceridemia in rats was diminished fed dietary cholesterol. Serum triglyceride concentration was markedly lowered by fish oil, whereas, this effect reached significance only using cholesterol-free diets. This was mainly associated with a reduction in VLDL triglycerides. Fish oil lowered HDL triglycerides only in rats fed the low-fat diet without cholesterol. Lipid components in the base diets did not influence serum and LDL phospholipids. Rats fed the high-fat/cholesterol-free diet had a higher VLDL phospholipid level than the other base groups. Irrespective of the base diet, phospholipid levels in serum and lipoproteins were markedly reduced by dietary fish oil. In conclusion, this study suggests that other dietary lipids should be considered when examining the hypolipemic effect of fish oil.