In-vitro autoradiography was utilized to compare the distribution of 2[125I]iodomelatonin binding sites or putative melatonin receptors in the gastrointestinal tracts of humans, guinea pigs, mice, rats, hamsters, rabbits, ducks, chickens, pigeons, and quail. In humans, binding was detected in the mucosa of the colon, caecum, appendix, and on their blood vessels but not in the ileum. In the other mammals, significant binding was only demonstrated in the mucosa of the rabbit rectum, mouse colon, mouse rectum, and guinea pig ileum. The distribution of 2[125I]iodomelatonin binding in the avian gut varied with species. In the esophagus, binding was present in the lamina propria and blood vessels of all four birds. However, only the lamina propria of the chicken and quail proventriculus and ventriculus showed positive binding. For the duodenum and ileum, binding was very strong in the duck lamina propria, weak in the chicken lamina propria, and absent in the quail. In contrast, the pigeon muscle layer was weakly positive. The most striking species difference was found in the caecum where the duck lamina propria showed very strong binding, while the chicken lamina propria was only weakly positive. Conversely, the caecal muscle layer was strongly positive in chicken and quail but negative in duck and pigeon. In the rectum, a similar but less intense pattern of distribution was observed. The tremendous diversity in the distribution of 2[125I]iodomelatonin binding sites in the gastrointestinal tract is in accord with the hypothesis that melatonin may serve different functions in the gut of different species.