Hamsters were given a diet containing fenofibrate (0.5% or 0.05%) or its metabolite, LF 2151 (0.15% or 0.015%) or a standard diet for a 3-week period. At the end of this period, the analysis of plasma lipids showed that the mean plasma triglyceride concentrations were not significantly different in the five groups of animals. The mean plasma cholesterol concentrations were significantly reduced in animals treated with both drugs but only when given at the high dosage. No consistent changes were noted in the liver weight/body weight ratio and the DNA content of the liver; the number of peroxisomes was increased in the hepatocytes of animals given fenofibrate at the high dosage. Liver homogenates were fractionated and the fractions rich in peroxisomes were used for assays of several enzymes involved in lipid metabolism. Compared with the control animals, activity of cyanide-insensitive fatty acyl-CoA (FA-CoA) oxidizing system was significantly increased by fenofibrate at the high dosage, carnitine acetyltransferase activity was markedly increased by both drugs at the high dosage and catalase activity remained unmodified. As there was a significant inverse correlation between the peroxisomal activity of FA-CoA oxidizing system and the plasma cholesterol concentrations, it is suggested that the increase of peroxisomal beta-oxidation activity can be involved in the hypocholesterolemic action of fenofibrate and LF 2151. This is further substantiated by the finding that fenofibrate and LF 2151 were present in the peroxisomal fraction only in hamsters displaying hypocholesterolemia and high activity of FA-CoA oxidizing system. The presence of fenofibric acid in the plasma of hamsters given LF 2151 suggested that hepatocytes are able to generate the parent drug from this metabolite, underlining that the pharmacokinetics of fenofibrate are rather complex in hamsters.