The effect on the intestinal mucosa of continuously infusing single amino acids, glycine, valine and histidine into the stomach and ileum was compared with saline and an amino acid mixture (AA) in rats fed by parenteral nutrition. After gastric infusion, glycine did not differ from saline, valine increased mucosa in the proximal segments and histidine alone increased mucosa in the proximal bowel equal to AA. After ileal infusion, all amino acids increased mucosa in the ileum. Valine and histidine, but not glycine, increased mucosa in the remote proximal small bowel. Therefore, regional differences occur in mucosal growth response to single amino acids.