The distribution of dopamine D1 receptors in the rat, labeled with [125I]SCH 23982, was studied using a quantitative in-vitro light-microscopic autoradiographic method. The binding of [125I]SCH 23982 to slide-mounted tissue sections and membrane preparations of prefrontal cortex was saturable, specific and of high affinity. Scatchard analysis revealed a Kd of 1.15 +/- 0.47 nM and Bmax of 8.76 +/- 0.34 fmol/mg tissue in prefrontal cortex membranes and a Kd of 1.27 +/- 0.14 nM and Bmax of 67.6 +/- 3.75 fmol/mg tissue in slide-mounted tissue sections at the level of the striatum. [125I]SCH 23982 was found to predominantly label D1 receptors, but a small fraction of the binding was to serotonin receptors. D1 receptors were found throughout the forebrain and were concentrated in the substantia nigra pars reticulata, accumbens nucleus, caudate putamen, entopeduncular nucleus, olfactory tubercle and the major island of Calleja. [125I]SCH 23982 binding to serotonin receptors was concentrated in the cortices, dorsal raphe, central gray, anterior hypothalamic area and the molecular cell layer of the cerebellum. Knowledge of the distribution of D1 receptors may increase our understanding of the role of D1 receptors in central nervous system dopaminergic function. Furthermore, data on the potential sites of interaction of [125I]SCH 23982 with serotonin receptors may help to understand the complex physiology and pharmacology of the primarily D1 selective compound.