Dehydroepiandrosterone sulfate prevented the increase in corticosterone level in rats induced by repeated exposure to stress. The mu-opioid receptor blocker naltrexone administered in a dose of 0.1 mg/kg 20 min before treatment with dehydroepiandrosterone sulfate abolished the effect of this agent. Dehydroepiandrosterone sulfate and naltrexone had no effect on rats after acute stress.