Ten male, normolipaemic, non-obese subjects were given clofibrate 2 g daily, fenofibrate 300 mg daily, bezafibrate 600 mg daily and probucol 1 g daily for eight days, in a crossover study with a wash-out period of 4-8 weeks between each drug regimen. Clofibrate, fenofibrate and bezafibrate caused a significant decrease in serum triglycerides, total cholesterol and LDL-cholesterol concentrations. Probucol caused a significant increase in serum LDL-cholesterol concentration. Serum HDL-cholesterol concentration was significantly increased by bezafibrate and significantly decreased by probucol. All drugs but probucol led to a significant rise in the activity of the plasma lipoprotein lipase; there was not a significant increase in the activity of plasma hepatic lipase after any drug. The activity of plasma lecithin: cholesterol acyltransferase was significantly increased by fenofibrate and probucol. Analysis of the correlations between serum lipids and plasma lipolytic enzymes suggests that the mechanism of the hypolipidaemic action of clofibrate and bezafibrate might be related to increased catabolism of triglyceride-rich particles; that of fenofibrate and probucol was less clear and might be multifactorial in origin.