This study investigates the role of endogenous opioids in the regulation of pain in humans. Two groups of healthy volunteers were subjected to different stress situations (cold pressor and arithmetic). In a double-blind design the changes in pain tolerance after stress were measured after an injection of either 0.8 mg naloxone or placebo. The cold pressor test raised the pain threshold in the placebo-treated group, but not in the naloxone-treated group. Mental arithmetic had no effect on pain perception. One can conclude therefore that physical stress may change pain perception depending upon the secretion of endogenous opioids.