The effect of hormones of hypothalamic-pituitary-adrenocortical system on pain sensitivity were studied in experiments on awake Sprague-Dawley males rats. Pain sensitivity was tested by tail flick reaction induced by thermal stimuli. Systemic glucocorticoids and ACTH injection increased the tail flick latency. The ACTH-induced analgesic effect was unaffected by deficiency of glucocorticoids production in pretreatment with pharmacological dose of cortisol but was fully eliminated by pretreatment with opiate antagonist naltrexone. These findings suggest that ACTH-induced analgesic effect is mediated by opiate receptors but not by glucocorticoids released in response to ACTH injection.