1. 1. The incorporation and turnover of amino acids into different protein fractions of the small intestinal mucosa of rats has been measured after the intravenous injection of leucine or mixed amino acids. 2. 2. The incorporation into the brush border fractions was delayed compared with other fractions and was compartible with the time required for the transfer of newly formed proteins from within the epithelial cell to the brush borders. 3. 3. After maximal labeling the half-life of the brush border was shorter (18 h) than the homogenate (31 h) or other subcellular fractions. 4. 4. The separated and purified disaccharides had an even faster turnover rate (11.5 h) than whole brush borders. Since migration per se cannot account for the turnover of disaccaharidases, control of disaccharidase turnover may not depend on cell formation in the intestinal crypts. It is suggested that control may be located at the villus level of the intestinal cells. 5. 5. Although the adult rat is known to have lower lactase activity than the neonatal animal, no differences in turnover between the disaccharidases were found to account for this reduced activity.