The effects of starvation (72 h) and refeeding with three liquid diets, differing only in the molecular form of the nitrogen source (whole whey proteins, WP; tryptic whey protein hydrolysate, WPH; and amino acid mixture, AAM), on the jejunal mucosal morphology and brush border enzyme activities (sucrase, S; maltase, M; and neutral aminopeptidase, NA) of male Wistar rats were studied. All three diets produced repair of the fasting-induced mucosal atrophy; the WP diet gave the most rapid growth with maximum villus height (VH) and protein content after 48 h (p less than 0.01). AAM gave the fastest and greatest stimulation of sucrase and maltase activities (p less than 0.01). There were no significant differences in NA activity. In control rats the WPH and AAM diets produced significantly greater villus height and disaccharidase activities than did the WP diet. Jejunal morphology and disaccharidase activities can be modified by the molecular form of alimentary protein and nutritional status interferes with these modifications.