The mouse dopamine D4 receptor gene was isolated from a genomic DNA library by plaque hybridization. The D4 receptor gene encoded an open reading frame consisting of 387 amino acids, interrupted by three introns. Distribution of the D4 receptor mRNA in brain regions and peripheral tissues of mice was examined by reverse transcription-polymerase chain reaction (RT-PCR). There was a remarkable expression of the receptor mRNA in various brain regions, showing the highest expression level in the cerebellum. Also, in the peripheral tissues a high level expression of the D4 mRNA was detected in the eye, adrenal gland and testes. We observed several differences in tissue distribution of the D4 mRNA in mice from that reported in other mammalian species.