We tested the hypothesis that the provision of glutamine and branched-chain amino acids would reverse the gut atrophy that accompanies parenteral nutrition. Three hundred seventy-five rats were randomized into 15 groups to receive either conventional parenteral nutrition, rat food, glutamine-enriched parenteral nutrition (0.5% to 2.5%), branched-chain amino acid-enriched parenteral nutrition (0.8% to 2.0%), or glutamine plus branched-chain amino acid-enriched parenteral nutrition (0.5%/0.4% to 1.25%/1/0%). When compared with effects of conventional parenteral nutrition, the infusion of either glutamine or branched-chain amino acids partially reversed, in a dose-dependent manner, atrophy of the small bowel as assessed by gut weight (p <.05), mucosal weight (p <.05), villous height (p <.05), crypt cell production rate (p <.05), and mucosal protein concentration (p <.05). There was no effect on the large bowel. These results suggest that the parenteral infusion of either glutamine or branched-chain amino acids partially reverses the small-bowel atrophy that is associated with the infusion of solutions of conventional parenteral nutrients.