Antagonist-precipitated withdrawal after acute opioid administration (acute physical dependence) is an interesting phenomenon in that the opioid abstinence syndrome is generally thought to develop only after prolonged exposure to opioid agonists. The purpose of this study was to examine further this phenomenon in humans by characterizing the antagonist dose-response function. The effects of i.m. naloxone (0, 0.1, 0.3, 1, 3, 10 and 30 mg/70 kg) were assessed 6 hr after single i.m. injections of morphine (18 or 30 mg/70 kg) in six subjects with a history of chronic opiate use. Naloxone reversed residual morphine effects, including miosis and respiratory depression. The degree of reversal was dose-related to 10 mg/70 kg of naloxone with no further increases at the highest naloxone dose. Simultaneously, observer ratings of withdrawal signs and subjective reports of withdrawal symptoms were increased in an orderly dose-related manner to 30 mg/70 kg of naloxone. Reversal of residual morphine effects and onset of precipitated withdrawal were evident by 5-min postnaloxone; peak effects occurred within 15 min. This study confirmed the occurrence of antagonist-precipitated withdrawal after brief opioid exposure in humans, demonstrated the rapid onset of withdrawal effects and characterized the naloxone dose-response function.