The polychlorinated biphenyl (PCB), Aroclor 1254, was given orally in peanut oil to pregnant Sherman rats once daily from day 7 to day 15 of organogenesis. Dose levels were 0, 10 and 50 mg/kg/day. No statistical difference was found between control and dosed groups with respect to the total weight of litters, the percentage of pups born dead or the survival rate to weaning. Residue levels of PCB-derived material were measured in the foetuses, in the milk ingested by sucklings and in tissues of weanlings by electron-capture gas-liquid chromatography. Liver enlargement was observed in weanlings from dosed rats, but not in those from controls. There was a significant increase in the relative liver weights of weanlings from dosed rats compared with those from controls and the livers of most of the exposed weanlings contained enlarged hepatocytes, accompanied, in some cases, by cytoplasmic vacuolization and bile-duct proliferation, particularly in the group given the higher dosage.