The time of appearance, morphology, and topographic distribution of somatostatin, neurotensin, bombesin, gastrin/CCK and serotonin immunoreactive cells during embryonic development were studied in the duck gastrointestinal tract by immunohistochemical methods. Somatostatin immunoreactive cells first appeared in the duodenum of duck embryos at 9 days of incubation (d.i.). They progressively appeared in the other segments at 19 d.i., and at hatching they were present in all gastrointestinal segments except for the caecum. At hatching, the antrum was the richest region in somatostatin endocrine cells, and the gizzard the poorest. Neurotensin immunoreactive cells were detected at 21 d.i. in the proventriculus, antrum, duodenum, and rectum; at 23 d.i. they were present in all the other segments. Bombesin immunoreactive cells were observed in the proventriculus at 17 d.i., and in the gizzard and antrum at 23 d.i. No cells were detected in the intestinal segments. Gastrin/CCK immunoreactive cells first appeared at 17 d.i. in the antrum region; at 21 d.i. they appeared in the small intestine and around hatching they were found in the other intestinal segments except for the proventriculus and gizzard. Serotonin immunoreactive cells appeared at 21 d.i. in the proventriculus, duodenum, and jejunum-ileum. At 23 d.i., they were present in all other segments. Our results show that the time of appearance of immunoreactive cells may be related to the general development of the gut wall in the duck and may reflect cell differentiation in the mucosa.