The radioligands [3H]SCH 23390 and [3H]spiroperidol were used to label dopamine D1 and D2 receptors, respectively, in rat brain slices. Striatal sections were incubated in one of various concentrations of radioligand in the presence or absence of (+)-butaclamol and the resulting labeling was determined by liquid scintillation spectrometry. Scatchard analyses of the data revealed KD values of 1.18 nM for D1 receptors and 0.33 nM for D2 receptors. Tissue sections taken from the entire rostrocaudal extent of the nucleus accumbens, as well as other brain regions, were then processed for autoradiographic analysis of D1 and D2 receptors using a radioligand concentration equal to 1.5 X KD. After apposing the slices to 3H-sensitive film, topographical differences among brain areas were analysed using a quantitative densitometry system which determined the absolute amount of ligand binding relative to calibrated optical density standards. The nucleus accumbens exhibited a rostral-to-caudal density gradient for both D1 and D2 receptors. For D1 receptors, the density was similar across most of the nucleus accumbens, although the most caudal portion examined had a lower density than rostral portions. In contrast, the density of D2 receptors exhibited a more gradual gradient across the entire rostrocaudal extent of the nucleus accumbens. There was no significant rostrocaudal density gradient of either D1 or D2 receptors in either the olfactory tubercle or caudate-putamen in the same tissue sections. A lateral-to-medial gradient of D2 receptors was also present in the nucleus accumbens. That is, while there was no difference in the density of D1 receptors between the lateral core and medial shell subdivisions, the shell had a lower density of D2 receptors than did the core. The density of D1 and D2 receptors in the mesolimbic and nigrostriatal systems was compared in groups of animals raised from 30 to 60 days of age in an impoverished condition, a group-caged condition or an enriched condition. While the brain weight of enriched condition animals was higher than impoverished condition animals, there were no significant differences in the density of D1 or D2 receptors among the different groups. Apparently, the densities of D1 and D2 receptors in the brain are resistant to differential rearing conditions.