Multiple D1 dopaminergic receptor subtypes have been postulated on the basis of pharmacological, biochemical, and genetic studies. We describe the isolation and characterization of a rat gene encoding a dopamine receptor that is structurally and functionally similar to the D1 dopamine receptor. The coding region, which is intronless, encodes a protein of 475 amino acids (Mr 52,834) with structural features that are consistent with receptors coupled to guanine nucleotide-binding regulatory proteins. The expressed protein binds dopaminergic ligands and mediates stimulation of adenylyl cyclase with pharmacological properties similar to those of the D1 dopamine receptor. The gene encoding the human homologue of this receptor subtype is located to the short arm of chromosome 4 (4p16.3), the same region as the Huntington disease gene. In striking contrast to the previously cloned D1 receptor, little or no mRNA for the receptor described here was observed in striatum, nucleus accumbens, olfactory tubercle, and frontal cortex. High levels of mRNA for this receptor were found in distinct layers of the hippocampus, the mammillary nuclei, and the anterior pretectal nuclei, brain regions that have been shown to exhibit little or no D1 dopamine receptor binding. On the basis of its properties we propose that this dopamine receptor subtype be called D1B.