Based on the sequence of a dopamine D2 receptor cloned from rat brain, we prepared a series of oligodeoxynucleotide probes. A mixture of these probes hybridized with a 2.6-kilobase species of mRNA extracted from several rat tissues including retina and, using in situ hybridization of these probes to cryostat sections of rat retina, they densely label the inner nuclear and outer plexiform layers. Labeling was also observed in the inner plexiform and ganglion cell layers. No hybridization was observed to the photoreceptor layers. A similar pattern of labeling was observed in monkey retina, indicating that the probes also hybridize with a homologous primate mRNA. The probes were used to screen a lambda gt10 library of human retina. A 2.5-kilobase clone was isolated, which encodes a protein that differs from the rat brain protein by 18 amino acids. The 5' and 3' untranslated regions of the human retinal cDNA were also strongly homologous with the rat brain cDNA. The clone was subcloned into the pCD-PS expression vector and transfected into COS-7 cells. The transfected cells bound [3H]-raclopride with a pharmacology expected of dopamine D2 receptors. These data indicate that D2 receptors expressed in the inner retina and outer plexiform layer have genetic identity with those expressed by brain and that the human and rat D2 receptors are derived from highly related genes.