Immunohistological localization of melatonin in the rat digestive system

The highest amount of melatonin (M), localized immunohistologically, was found in rectum, decreasing in the order: colon, duodenum, caecum, esophagus, stomach, ileum and jejunum. No M was found in the liver, spleen or pancreas. The distribution of M corresponds with the localization of serotonin-producing argentaffin cells.