Atropisomers of the polychlorinated biphenyls 2,2',3,4,4',6-hexachlorobiphenyl (II) and 2,2',3,3',4,4',6,6'-octachlorobiphenyl (III), stable to racemization under physiological conditions, were administered to immature male Sprague-Dawley rats. The racemic hexachlorobiphenyl (II) was found to be a potent (phenobarbital-type) inducer, whereas (+)-II and (-)-II, administered at 100 mumol/kg, showed clearly differing potencies as inducers with (+)-II enhancing aminopyrine N-demethylase, aldrin epoxidase and cytochrome P-450 content more potently than (-)-II. In contrast, the racemic octachlorobiphenyl (III) and its individual enantiomers were only weak phenobarbital-type inducers of cytochrome P-450, and the enantiomers of III were equally (weakly) potent. Separate studies conducted to investigate pharmacokinetic influences on the differential potency of the enantiomers of II showed that after 5 days the concentration of (+)-II in the liver was twice as high as that of its antipode. Therefore, enantioselectivity in disposition as well as in recognition may be responsible for the differential potency seen.