Blood serum melatonin concentrations were measured in male Wistar rats exposed to acute water-immersion emotional stress (ES) during intraperitoneal injections of physiological saline or different doses of melatonin. ES increased blood melatonin levels in rats receiving physiological saline 1 hour before or just after ES. When physiological saline was administered just ES, blood melatonin concentrations remained unchanged. The blood levels of melatonin both in control and stressed animals injected with exogenous melatonin were higher than those in rats given physiological saline alone. Immersion ES decreased blood melatonin concentrations in rats receiving different exogenous melatonin doses just before ES. However, in rats given melatonin, 1.0 mg/kg, after ES, its blood levels after ES was higher than those in the animals unexposed to stress. The findings suggest that it is just exogenous rather than endogenous melatonin that is consumed in rats injected with its different doses just prior to immersion ES.