We previously reported that daily exposure for 4 days to an inescapable form of footshock stress, known to cause opioid-mediated analgesia, suppressed the cytotoxic activity of splenic natural killer (NK) cells in rats. Similarly, daily injection of high doses of morphine (greater than or equal to 30 mg/kg) for 4 days also suppressed splenic NK cell activity. We now report that a single exposure to the opioid form of footshock stress or a single high dose of morphine induces suppression of splenic NK cell cytotoxicity. This effect is evident 3 h after treatment, returning to normal by 24 h. Morphine-induced NK suppression is evident in both male and female rats, is blocked by the opiate antagonist naltrexone, and develops tolerance. Morphine-induced NK suppression is seen in cells derived simultaneously from the spleen, bone marrow, and peripheral blood, suggesting that this suppression does not result from a selective egress of NK cells from the spleen.