The ventral tegmental area (VTA) and its dopaminergic projections appear to mediate some of the rewarding properties of opiates, cocaine, and other drugs of abuse. In a previous study, we demonstrated that chronic morphine and cocaine exert common actions on tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, in this dopaminergic brain reward region (Beitner-Johnson and Nestler, 1991). In the present study, we investigated the effects of chronic morphine and cocaine on other phosphoproteins in the VTA by back phosphorylation and two-dimensional electrophoretic analysis. It was found that a number of phosphoproteins, in addition to tyrosine hydroxylase, were regulated similarly by the two drug treatments in this brain region. Several of these morphine- and cocaine-regulated phosphoproteins were identified as neurofilament (NF) proteins. Chronic, but not acute, administration of either morphine or cocaine was found to decrease levels of the three NF proteins, NF-200 (NF-H), NF-160 (NF-M), and NF-68 (NF-L), by between 15% and 50% in the VTA by back phosphorylation, immunolabeling, and Coomassie blue staining. Such regulation of NF proteins was selective, in that no detectable changes were observed in the levels of eight other major cytoskeletal or cytoskeletal-associated proteins analyzed. Furthermore, NF levels were not altered by chronic treatment with either imipramine or haloperidol, two psychotropic drugs without reinforcing properties, or by chronic stress. Morphine and cocaine regulation of NFs showed regional specificity, as NF levels were not altered in the substantia nigra, or other parts of the brain or spinal cord, by these drug treatments. NFs are thought to function as determinants of neuronal morphology and to be associated with axonal transport. Thus, decreased NF levels in the VTA in response to chronic morphine and chronic cocaine could lead to drug-induced alterations in the structural and functional properties of this brain region, which may represent, in turn, part of a common biochemical basis of morphine and cocaine addiction and craving.