The incorporation of thymidine methyl-3H (3H-TdR) into deoxyribonucleic acid (DNA) was measured in the intestine of diabetic rats. Animals were allowed food ad libitum or were pair-fed. Diabetes was induced with streptozotocin. In the pair-feeding experiment, there was a decrease in 3H-TdR incorporation into DNA in the intestine of diabetic rats, and the mucosal weight did not differ significantly from controls. However, when diabetic rats were allowed food ad libitum, there was a significant increase in DNA synthesis in the intestine. The mucosal weight of the diabetic animal was 62 per cent greater in the jejunum and 31 per cent greater in the ileum than in the control segments. It is concluded that a decrease in proliferative cellular activity occurs in the intestinal mucosa of diabetic rats but hyperphagia leads to a compensatory increase in DNA synthesis in these animals.