The ontogeny and distribution of nerve cell bodies and fibers which are substance P-like (SPL) immunoreactive have been studied in the duck gastrointestinal tract by means of immunohistochemistry. The appearance of SPL immunoreactivity was observed in cell bodies of the oesophagus, proventriculus, gizzard and antrum region at 6 days of incubation. At 9 days of incubation SPL immunoreactive nervous structures were observed in the whole length of the intestine. In each area of the gut, SPL immunoreactivity was initially observed in cell bodies of the myenteric plexus and then of the submucous plexus. Successively, immunoreactive fibers appeared in the same plexuses and also in the circular muscle, longitudinal muscle and muscularis mucosae plexus. Only around hatching were SPL immunoreactive nervous structures seen in the mucous plexus. It was observed that in adult ducks, immunoreactive nerve cell perikarya were less numerous than in embryos and in newly-hatched ducks. The pattern of distribution of SPL immunoreactivity in adult ducks was rather similar to that of the chick and other vertebrate species.