Morphine or butorphanol was continuously infused into cerebroventricle (i.c.v.) with the rate of 26 nmol/ μl/h for 3 days, and the withdrawal from opioid was rendered 7 hrs after the stopping of infusion. The expression of physical dependence produced by these opioids was evaluated by measuring the naloxone- precipitated withdrawal signs. The withdrawal signs produced in animals dependent on butorphanol (kappa opioid receptor agonist) were similar to those of morphine (mu opioid receptor agonist). Besides the behavioral modifications, opioid withdrawal affected G protein expression in the central nervous system. The G-protein α-subunit has been implicated in opioid tolerance and withdrawal. The effects of continuous infusion of morphine or butorphanol on the modulation of G protein α-subunit mRNA were investigated by using in situ hybridization study. In situ hybridization showed that the levels of Gαs and Gαi were changed during opioid withdrawal. Specifically, the level of Gαs mRNA was decreased in the cortex and cerebellar granule layer during the morphine and butorphanol withdrawal. The level of Gαi mRNA was decreased in the dentate gyrus and cerebellar granule layer during the morphine withdrawal. However, the level of Gαi mRNA was significantly elevated during the butorphanol withdrawal. These results suggest that region-specific changes of G protein α-subunit mRNA were involved in the withdrawal from morphine and butorphanol.