The significance of disturbances of lipid metabolism caused by xenobiotic acyl-CoAs as possible causes of peroxisomal proliferation has been studied with the enantiomers of 2-phenylpropionic acid (2-PPA), the (R)-enantiomer of which is converted to the acyl-CoA in rats while its (S)-antipode is not. rac-2-PPA (250 mg/kg/day ip x 3) was shown to be an hepatic peroxisomal proliferator in male Sprague-Dawley rats on the basis of increases in microsomal cytochrome P-450 content and lauric acid hydroxylation and hepatic CN(-)-insensitive palmitoyl-CoA oxidation, a peroxisomal marker activity, while electron microscopy revealed a rise in the peroxisome/mitochondria ratio in hepatocytes. Further studies established the dose-response relationships for these biochemical changes. The (R)- and (S)-enantiomers were administered at a dose of 50 mg/kg/day ip x 3 and both were peroxisome proliferators of very similar potency. The effects of 100 mg/kg/day ip x 3 of the racemate, a dose giving ca. 75% of maximal response, were essentially additive of those of 50 mg/kg/day ip x 3 of its two component isomers. The stereoselectivity of acyl-CoA formation from the enantiomers of 2-PPA was confirmed by their differential inhibition of microsomal palmitoyl-CoA synthesis. Taken together, these data indicate that it is very unlikely that the acyl-CoA of 2-PPA plays any role in the peroxisomal proliferation which this compound causes in the rat.