The present experiment was designed to examine whether the acute sensitization to naltrexone that is induced by a single dose of morphine could be blocked by pretreatment with naloxone. Food-deprived male Sprague-Dawley rats were trained to respond for food on a multiple-trial fixed interval 3-min schedule. Reinforcement was contingent upon a response within a 10-s limited hold period following a fixed interval of 3 min. A trial consisted of three fixed intervals separated by a 10-min timeout period during which responses were not reinforced. The rate decreasing effects of the opioid antagonist naltrexone were determined by cumulative dosing. Pretreatment with morphine (3.0 mg/kg, SC) resulted in a 70-fold increase in sensitivity to the response rate decreasing effect of naltrexone compared with saline pretreatment. The increased sensitivity was dose-dependently blocked by naloxone administration 10 min before morphine. The blockade by naloxone was overcome by increasing the pretreatment dose of morphine to 10 mg/kg. The results provide further evidence that acute agonist-induced sensitization to the rate-reducing effects of naltrexone is mediated by opioid receptors.