Imunocytochemical techniques were used to determine whether agonist-induced activation of mu-opioid receptors alters the number and distribution of mu-opioid receptor-positive cells in the rat cerebral cortex. In untreated rats, mu-opioid receptor immunoreactivity was localized to neuronal perikarya and dendrites and to neuropilar punctate structures. mu-Opioid receptor-positive neurons were mostly in layers II and III and exhibited a bipolar or bitufted morphology. In rats treated with the mu-opioid receptor agonist etorphine (0.1mg/kg intraperitoneally) and perfused after different survival periods, there was an enhancement of immunostaining for mu-opioid receptors observed at 15min, reaching a maximum at 60min, and which returned to normal at 480min. Etorphine-induced effects included an increase in the intensity of cellular and neuropil staining; statistical analysis showed that the number of mu-opioid receptor-positive cells in etorphine-treated groups was significantly higher than in controls or saline-treated rats. In animals that received both etorphine and the mu-opioid receptor antagonist naloxone, the pattern of mu-opioid receptors immunoreactivity was similar to that of untreated animals. This study shows that the number of mu-opioid receptor-positive cells is significantly increased following etorphine treatment and suggests that agonist treatment may be exploited to increased immunostaining of mu-opioid receptors and also of other G-protein coupled receptors.