The effects of polychlorinated biphenyls (PCBs) on the development of several intestinal and serum marker enzymes have been studied. The three congeners 4‐monochloro (1‐CB), 3,4,3’,4'‐tetrachloro (4‐CB), and 2,4,5,2’,4’,5'‐hexachloro (6‐CB) biphenyl were administered orally to pregnant rats on d 8, 11, 13, 15, and 18 of gestation. 1‐CB and 6‐CB were intubated at doses of 30 mg/kg·d (total dose, 150 mg/kg) and 4‐CB was administered at 3 mg/kg·d (total dose, 15 mg/kg). Levels of intestinal alkaline phosphatase, monoamine ioxidase, and Na+,K+‐adenosin‐5'‐etriphosphatase and levels of serum alkaline phosphatase, sorbitol dehydrogenase, and ß‐hydroxybutyrate dehydrogenase were measured in the dams after weaning and in their offspring at —1, 6, 20, and 55 d of age. Intestinal alkaline phosphatase activity was elevated at the later postnatal stages in the 1‐CB group and depressed at 55 d in the 4‐CB group, whereas serum alkaline phosphatase levels were markedly depressed prenatally and postnatally in the 4‐CB and 6‐CB groups, respectively. Intestinal monoamine oxidase levels were markedly increased in the 6‐CB group at —1, 6, and 20 d of age and significantly depressed in the 4‐CB animals at —1 and 55 d of age. There was an increase in monoamine oxidase activity in the 4‐CB group at 6 d. The 1‐CB group exhibited depression of monoamine oxidase levels at 6 d and elevation at 20 and 55 d. Intestinal Na+,K+‐A TPase levels were elevated throughout development in the 1‐CB animals and at —1 and 6 d in the 4‐CB group. The 6‐CB animals showed elevated levels of Na+,K+‐A TPase only at 6 d. Serum ß‐hydroxybutyrate dehydrogenase and sorbitol dehydrogenase were induced prenatally in the 4‐CB animals but enzyme activities decreased to normal by 55 d of age. Significant depression of activity was evident in the 1‐CB and 6‐CB groups at —1 d in both enzymes.