Experiments were conducted to compare directly the effects of clofibrate (ethyl 2-(p-chlorophenoxy)isobutyrate) and its analogs on serum lipids and on the rate-limiting enzyme of the hepatic peroxisomal beta-oxidation pathway, fatty acyl-coenzyme A oxidase. Clofibrate feeding (5 g/kg diet) led to 8-10-fold increases in enzyme activity in 7 days. The enzyme remained elevated during the 28-day course of the experiment. The treatment did not lead to a lowering of serum cholesterol or triacylglycerol at any time during the experiment. In separate experiments rats were given clofibrate or one of five analogs by gastric intubation for 4 consecutive days. Four of the five analogs studied caused a significant lowering of serum triacylglycerol to about 50% of pretreatment level; another analog and clofibrate itself did not significantly affect serum triacylglycerol levels. None of the five analogs caused an induction of hepatic fatty acyl-CoA oxidase, while clofibrate treatment led to a 3-4-fold increase in enzyme activity. The results demonstrate a complete dissociation between the hypolipidemic action of these compounds and the induction of peroxisomal beta-oxidation. Thus it appears that the two phenomena are mechanistically unrelated.