Only P450 2D mRNA could be detected in Northern blot analysis of rat brain poly(A)+ RNA using probes from eight different P450s involved in drug metabolism. Four cerebral P450 cDNAs were isolated from a rat brain cDNA library using rat P450 2D2 cDNA as a probe, three of which were identified to rat P450 2D4. The expression of 2D4 mRNA was investigated by the reverse transcriptase-polymerase chain reaction and southern blot analysis using an oligonucleotide probe specific for P450 2D4. P450 2D4 mRNA was expressed at the high level in brain. Furthermore, it was detected in all other tissues examined, although to a lesser extent except in the liver and small intestine. However, total mRNAs of other P450 2D members (2D1, 2D2, 2D3, and 2D5) were mainly expressed in liver. The total level of expression of these mRNAs varied markedly among tissues. These results suggest that P450 2D4 may have a specific physiological function in the brain.