To understand the possible involvement of dopamine receptors in the pathogenesis of various neurological disorders, we have cloned and sequenced a dopamine D2A receptor gene from the mouse. A mouse genomic library was screened with probes derived from the published sequence of a rat D2A receptor cDNA. Using restriction endonuclease mapping, Southern blotting, and DNA sequencing, we have determined the cDNA sequence and genomic organization of the mouse D2A receptor gene. Unlike other guanine nucleotide-binding protein-coupled receptors, but similar to its rat and human counterparts, the mouse D2A receptor gene has seven introns and spans at least 30 kb of genomic DNA. The mouse D2A sequence shows 99% amino acid homology with the rat and 95% amino acid homology with the human sequence. As would be predicted, sequence differences are significantly more frequent outside of the hypothesized transmembrane spanning domain regions of the protein. Using the polymerase chain reaction with primers made from neighboring exons, we have identified two alternatively spliced D2A transcripts in the mouse. However, in contrast to the other species studied, the mouse expresses primarily the mRNA representing the larger, 444-amino-acid form of the receptor. Mouse pituitary expresses only the mRNA of the 444-amino-acid form of the D2A receptor. Hence, the mouse may offer the best model to study the in vivo physiology of the long form of the D2A receptor.