In rats subjected to restraint and exposure to cold, naloxone did not significantly influence the increased serum concentrations of corticosterone or the incidence of stress ulceration, but it significantly reduced the severity of gastric lesions. These findings suggest that endogenous opioids released during stress may contribute to the pathogenesis of stress ulceration. They also support the theory that the adrenocorticosteroids are unimportant aetiological factors in stress ulcer formation.