The effects on plasma lipids of daily intraperitoneal injections of 4 mg of melatonin (N-acetyl-5-methoxytryptamine) for 10-27-day periods were examined biochemically and morphologically in rats fed regular and high-cholesterol (1% cholesterol, 0.5% cholic acid) diets. Melatonin administration had no significant effect on plasma lipids and lipoproteins in the rats on a normal diet but blunted the effects of a high-cholesterol diet on these parameters. No effects of melatonin on lipase activity were noted. Melatonin also diminished the fatty infiltration in the liver of animals on the high-cholesterol diet. The high-cholesterol diet produced major increases in VLDL and LDL cholesterol and protein content, and decreases in HDL cholesterol and protein. Melatonin decreased the extent of this plasma lipoprotein increase, although it did not completely prevent the phenomenon. Therefore, the effect is thought to be quantitative and not qualititative in nature.