Comparative studies on small intestinal development in artificially reared (AR) and in mother-fed (MF) rats were carried out. When pups at 12 days of age were reared by continuous intragastric infusing of a cow milk formula (CMF), they gained weight equal to MF rats until 20 days; their small intestine weight was approximately 40 and 23% greater than that of MF rats at 16 days and 22 days of age, respectively. Autoradiographic studies indicated that the intestinal DNA synthetic index was 74-85% higher in AR than in MF rats at 16 days; similarly intestinal crypt depth and duodenal villus length were significantly higher in AR rats. The developmental pattern of duodenal alkaline phosphatase activity was similar in both AR and MF rats. In contrast, AR rats showed precocious decrease of lactase activity in the ileum at 16 days and in the jejunum at 18 days; but total lactase activity in the intestine was not different from MF rats at 16 days. Jejunal sucrase activity in AR rats was precociously induced and elevated. It was concluded that the intestine of AR rats undergoes structural and enzymic modifications. The physiological significance of these changes remains to be elucidated.