We have recently demonstrated that the pineal neurohormone melatonin exerts important immunoregulatory functions. We now report that exogenous melatonin counteracts completely the effect of acute anxiety-restraint stress on thymus weight and antibody response to sheep red blood cells (SRBC). In addition, administration of melatonin in the evening prevented paralysis and death of mice infected with sublethal doses of encephalomyocarditis virus (EMCV) after acute stress. The anti-stress activity of melatonin was present in mice injected with T-dependent antigens, and it was abolished by the contemporary administration of the specific opioid-antagonist naltrexone. This suggests that melatonin exerts its remarkable anti-stress effect on antigen-activated cells via an opiatergic mechanism. These findings have important implications at both basic and clinical levels. They provide a new approach to a possible physiological 'up-regulation' of the immune response under virus- and/or stress-related immunosuppression.