This study compared the direct effects and acute physical dependence of butorphanol and morphine, opioids with differing actions at mu versus kappa receptors. Six non-dependent heroin-using volunteers were exposed to six conditions in a within-subject, Latin square design using double-blind procedures. In each session, agonist effects of single i.m. injections of butorphanol (3 and 6 mg/70 kg), morphine (15 and 30 mg/70 kg), lorazepam (4 mg/70 kg) or saline were evaluated. Butorphanol and morphine produced effects of comparable magnitude on miosis and reports of 'any drug effect'. Volunteers reported dysphoria, confusion and sedation after butorphanol, subjective effects that overlapped with those of lorazepam, whereas morphine produced euphoria and stimulation. Acute physical dependence (i.e. precipitated withdrawal responses to naloxone 10 mg/70 kg i.m. administered 6 h after each treatment) significantly increased after 30 mg/70 kg morphine but not after butorphanol treatments. These differences in naloxone sensitivity are likely due to differences in opioid receptor (mu versus kappa) activity, affinity and efficacy of these compounds. Pharmacological ramifications of these results are discussed.