The effects of dexamethasone and naloxone on analgesia induced by swimming (3 min, 20 degrees C) were studied in the 6th and 7th generations of adrenalectomized and intact mice selectively bred for high (HA) and low (LA) postswim analgesia. Swim-induced analgesia in intact HA animals was significantly reduced by naloxone and dexamethasone while in LA mice these two compounds were ineffective. Naloxone ability to reverse adrenalectomy-caused swim analgesia increase was much greater in HA than in LA mice. In both intact and adrenalectomized HA animals dexamethasone and naloxone decreased postswim analgesia to the level observed in LA mice. It is suggested that selective breeding for high and low swim analgesia modified the extent of pituitary-adrenal axis involvement in the generation of stress-induced analgesia.