In this study we demonstrate that a 51-kDa phosphoprotein, previously identified as morphine regulated and showing different basal levels among rat strains, is glial fibrillary acidic protein (GFAP). Chronic morphine increased levels of GFAP immunoreactivity by > 70% in the ventral tegmental area (VTA) of outbred Sprague-Dawley rats. This increase in GFAP content was not observed in rats that were treated concomitantly with morphine and naltrexone, an opiate receptor antagonist, and did not occur in response to a single acute injection with morphine. No alterations in GFAP levels were observed in response to chronic morphine in several other regions of the CNS studied, including the substantia nigra, locus coeruleus, cerebral cortex, and spinal cord. There were also inherent differences in levels of GFAP immunoreactivity in the VTA of drug-naive Fischer 344 and Lewis rats, two inbred rat strains that differ in their relative preference for morphine and other drugs of abuse. The VTA of drug-naive Lewis rats contained more than twofold higher levels of GFAP compared with drug-naive Fischer rats. This strain difference was also apparent in the locus coeruleus but not in several other brain regions or in spinal cord. Because the mesolimbic dopamine system is thought to play a critical role in mediating the reinforcing properties of opiates and other drugs of abuse, it is possible that the opiate induction of GFAP and inherent Lewis versus Fischer strain differences in GFAP levels in the VTA may be related to the reinforcing and/or addictive properties of opiates mediated by this brain region, as well as to genetic differences in drug preference.