In order to investigate the effect of food deprivation on the levels of indoles in the brain and the gastrointestinal tissues, we have determined tissue levels of tryptophan (TRP), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), and melatonin in the brain and the gastrointestinal tract (GIT) of mice on ad libitum diet as well as in mice deprived of food for 24 and 48 hr. The reduction of food intake 1) had no effect on TRP levels in the brain, but increased TRP concentrations in the stomach and the gut, especially in the colon; 2) decreased 5-HT levels in the brain, but increased values in the stomach and the intestines; 3) decreased 5-HIAA levels in the brain, but increased them in the stomach and the intestines; 4) did not change 5-HT conversion to 5-HIAA in the brain, stomach, and the jejunum, but increased the conversion in the ileum and colon and; 5) increased melatonin levels in all tissues investigated, particularly in the stomach and the brain. The changes of indole levels induced by food deprivation were compared to their known function in the brain and the individual segments of the GIT. A possible serotonin-melatonin antagonism in the brain and GIT function is considered.