- 1. Compared to night time values, levels of melatonin (M), determined by specific RIA, were lower in the brain and the duodenum-jejunum segment of mice sacrificed in mid-photophase. In other parts of the gut no significant diurnal difference was observed. 2. Highest daytime levels were detected in the stomach (569 pg/ml), lowest in the brain (62 pg/ml). 3. Preliminary studies indicate the presence of specific binding sites for [125I]iodo-melatonin in the colon, ileum, jejunum, stomach and brain. These sites, which exhibit an affinity of about 1 nM, may be involved in mediating the gastrointestinal and central effects of melatonin in this species.