Chronic treatment with the opioid antagonist naltrexone induces functional supersensitivity to opioid agonists, which may be explained by receptor up-regulation induced by opioid receptor blockade. In the present study, the levels of opioid receptor subtypes through the brain of mice were determined after chronic naltrexone treatment using quantitative in vitro autoradiography. This is the first complete mapping study in mice for µ-, δ- and κ-opioid receptors after chronic naltrexone exposure. Treatment with naltrexone clearly induced up-regulation of µ- (mean 80%) and, to a lesser extent, δ-opioid receptors (mean 39%). The up-regulation of µ- and δ-opioid receptors was evident throughout the brain, although there was variation in the percentage change across brain regions. In contrast, consistent up-regulation of κ-opioid receptors was observed in cortical structures only and was not so marked as for µ- and δ-opioid receptors. In noncortical regions κ-opioid receptor expression was unchanged. Taken together, the present findings suggest opioid receptor subtype-selective regulation by chronic naltrexone treatment in mice.