Replacement of villous epithelium in the small intestine of sheep and calves was studied by tritiated thymidine (3-H-TdR) autoradiography. Sheep were less than 1 day old, 3 weeks old, 3 months old, or more than 1 year old, and calves were less than 1 day old or 3 months old. In both species, there was marked variation in the location of labeled epithelial cells on villi among individuals from the same age group studied at the same time after administration of a single intravenous dose of 3-H-TdR. Labeled epithelial cells were confined to the proximal half of the villi at 48 hours after thymidine exposure in the animals, except some 3-month-old individuals from both species. Labeled epithelial cells had reached or were near the tips of the villi in these individuals at 48 hours. In both species, villi were longer and crypts were shorter in the newborn animals than in the older animals. The data were interpreted to indicate that replacement times were well in excess of 48 hours in neonates from both species and decreased somewhat with age.