Melatonin administration reduces the severity of dextran sodium sulphate (DSS)-induced colitis in mice. After 7 weeks of daily intraperitoneal melatonin administration (150 micrograms/kg), rectal bleeding and occult blood was eliminated in all mice in which colitis was induced by DSS. In addition the frequency and severity of focal lesions in the mucosa was dramatically reduced. Furthermore, weight loss and higher food consumption observed in DSS-treated mice was reversed in DSS-treated mice injected with melatonin. All treated groups exhibited significant alterations in goblet cell distribution as a result of DSS or melatonin administration. Surprisingly, serum melatonin levels were more than 10 times higher in mice that received DSS as compared to controls. The significant improvement in the conditions of melatonin-treated mice might be due to its effect on the smooth muscles of the colon, the blood supply in the mucosa, its capability as an antioxidant and scavenger of free radicals, or its effect on the immune system of the gut. The higher plasma levels of melatonin in DSS-treated mice might be due to a stress-induced increase in the production of gastrointestinal (GIT) melatonin.