Butorphanol is a synthetic opioid agonist/antagonist analgesic agent that mainly exerts its effects through kappa-opioid receptors. It has been demonstrated that kappa-opioid receptors preferentially mediate the development of physical dependence upon butorphanol and the associated withdrawal syndrome. However, it is not fully understood whether or not nNOS-containing neurons in the various brain regions play an important role in butorphanol withdrawal. Therefore, this study was conducted to determine whether the selective nNOS inhibitor, 7-NI, modifies the development of butorphanol withdrawal and changes of nNOS expressions in different brain regions in physically butorphanol-dependent rats. The first part of the study focused on withdrawal behaviors in male Sprague-Dawley rats. Physical dependence was induced by a 72-h i.c.v. infusion with butorphanol (26 nmol/mixrol/h) and withdrawal was subsequently precipitated by i.c.v. challenge with naloxone (48 nmol/5 microl/rat) 2 h after termination of the butorphanol infusion. The butorphanol/7-NI coadministration group showed a significant decrease in several signs of withdrawal such as teeth chattering, as compared with the butorphanol-treated group. In the second part of the study, immunohistochemical analysis was performed to determine the expression of nNOS in the various brain regions. In the butorphanol/7-NI coadministration group, the number of cells labeled for nNOS was significantly lower in the various brain regions (including the caudate putamen, nucleus accumbens, and hippocampus) than in the butorphanol group. Therefore, 7-NI decreased in butorphanol-induced physical dependence and nNOS expression. Taken together, these findings suggest that the nNOS system is involved in the development of butorphanol-induced physical dependence, and 7-NI has potential clinical application as a candidate for the treatment of opioid withdrawal syndrome.