1. The ability of rat ileal enterocytes to take up alanine and lysine before and after proximal resection of the small intestine has been assessed using both autoradiographic and dual-isotope methods of analysis. 2. The length of individual villi was approximately doubled after resection. Alanine uptake measured in the presence or absence of Na, represented per cm2 mucosal surface area, decreased following intestinal resection. 3. Alanine and lysine uptake were confined to villus tip enterocytes in both control and ileal remnants. The net effect of intestinal resection was to increase amino acid uptake calculated per individual villus or per unit length of intestine. 4. Adaptational changes occurring as a result of resection included a shortening of the time needed for enterocytes to reach the stage where they first began to absorb amino acids and a doubling of the rate at which absorption increased during the later stages of enterocyte differentiation. 5. It is suggested that the physiological response to intestinal resection can best be appreciated by studying events taking place within individual enterocytes. The way in which adaptational changes become organized within the mucosa remains to be elucidated.