Immunoreactivity for substance P (SP-LI) and vasoactive intestinal polypeptide (VIP-LI) in the stomach, duodenum, jejunum and ileum of young (3-month-old), adult (12-month-old) and old (28-month-old) male Wistar rats were determined by radioimmunoassay. SP-LI and VIP-LI did not show changes in the stomach or in the small intestine portions investigated in adult in comparison with young rats. In old rats SP-LI levels were unchanged in the stomach and in the duodenum, reduced in the jejunum and increased in the ileum. VIP-LI concentrations were unchanged in the stomach, and remarkably reduced in the duodenum, jejunum and ileum of old rats. Since the two neuropeptides investigated are localized primarily within different intrinsic gastrointestinal neurons, the present findings suggest that the various populations of gastrointestinal neurons are affected in a different manner during aging.