The influence of the mesencephalic dopaminergic projections on the neurons within the basal forebrain and prefrontal cortex is not well understood although it has been intensely investigated. The purpose of this study was to evaluate the expression of Fos-like and FRA-like (Fos Related Antigens) immunoreactivity (IR) as a qualitative and quantitative marker of neuronal activity within the mesolimbic and mesostriatal dopamine terminal fields. Following the administration of apomorphine (5.0 mg/kg S.C.), a rapid increase in FRA-IR, accompanied by Fos-IR, was observed within the striatum in a patchy distribution. Apomorphine also induced the expression of FRA-IR within the nucleus accumbens, cortex, septum, and the islands of Calleja complex. This broad pattern of activation contrasts with the limited expression of Fos-IR and FRA-IR within the dorsolateral striatum, dorsomedial shell of the nucleus accumbens, and cingulate cortex following haloperidol administration (2.0 mg/kg, S.C.). Finally, it was observed that nuclei expressed Fos-IR rapidly and transiently within the striatum following haloperidol, whereas the number of FRA-IR nuclei remained elevated but changed in distribution and intensity over time. In conclusion, different regions within the dopamine terminal fields express varying concentrations of Fos-IR and FRA-IR after stimulation or blockade of dopamine receptors. These data indicate that Fos, as well as selective FRAs, can be used to delineate populations of neurons with altered metabolic activity resulting from the administration of dopaminergic agents. Furthermore, the data support the concept of segregated mesostriatal and mesolimbic projections, in particular the division of the nucleus accumbens into the shell and core compartments.