Daily cocaine administration has been shown to alter G proteins in mesolimbic nuclei, and these changes have been implicated in the initiation and expression of behavioral sensitization. To evaluate the robustness of changes in G proteins induced by daily cocaine treatments capable of producing behavioral sensitization, the levels of Gi1 alpha, Gi2 alpha, Go alpha, Gs alpha, and G beta protein were measured by immunoblotting at 1 hr after an acute injection of cocaine or saline given 1 or 14 days following the last injection of daily cocaine or saline. A significant decline in Gi1 alpha was seen in the nucleus accumbens at 14 days following daily cocaine administration regardless of whether they received an acute challenge with cocaine or saline 1 hr prior to decapitation. No alterations were observed in the ventral tegmental area, substantia nigra, dorsolateral striatum, or prefrontal cortex in the levels of Gi1 alpha, Gi2 alpha, or Go alpha. No change in G protein immunoreactivity was measured in the nucleus accumbens or ventral tegmental area of rats decapitated 1 hr after discontinuing daily cocaine. The possibility that a long-term change in Gi1 alpha in the nucleus accumbens may be related to cocaine-induced behavioral sensitization is discussed.