This study assessed the effects of i.m. naloxone (10 mg) 6 hr after acute i.m. injections of morphine (0, 1, 3, 5.6, 10 and 17 mg). Naloxone reversed residual morphine-produced respiratory depression, miosis and subjective reports of drug "high." In addition, naloxone precipitated signs and symptoms characteristic of opioid withdrawal. Subjective report measures of "bad" drug effects and specific opioid withdrawal symptoms increased as a function of morphine pretreatment dose, as did observer-rated signs of withdrawal. Yawning was the most prominent observed sign, whereas yawning and irritability were the most consistently reported subjective symptoms. Peak withdrawal effects were seen within 15 min post-naloxone. The results of this study confirm previous reports of acute physical dependence in man and extend those findings by demonstrating a morphine dose-response function.