The postnatal development of NADPH-cytochrome c reductase, UDP-glucuronosyltransferase, selenium-dependent glutathione peroxidase, glutathione-S-transferase activities, total non-protein sulfhydryls, and cytochrome P-450 contents was compared in small intestine and liver of male Wistar rats from 18 to 60 days of life. In the intestine, main age-related changes affected conjugation enzymes which were increased rapidly by a factor of 4 during the weaning time and remained unchanged thereafter. Development in the rat was accompanied by changes in cytochrome P-450 content and NADPH-cytochrome c reductase activity only in the liver. In both organs, glutathione concentration and selenium-dependent glutathione peroxidase activity were not significantly modified between 18 and 60 days. In the liver, we observed discontinuities in the developmental pattern of UDP-glucuronosyltransferase, and the ontogenesis of glutathione-S-transferase activities differed according to the substrate metabolized.