The small and large intestines of piglets gained rapidly in weight and length during the first 24 h after birth, much more rapidly than during the next 9 days. In the jejunum, which was particularly studied, both muscle and mucosa increased in weight, but the mucosa more than the muscle, and there was a considerable increase in the amount of protein in it. Some of this may have been colostral protein in process of being absorbed. The activities of lactase and acid phosphatase in the jejunal mucosa as a whole increased during the first 24 h in suckled animals, but because of the large increase in the amount of mucosa and of protein in it the enzyme activities, expressed per gram protein, fell.