CYP2D1, 2D2, 2D3, and 2D4 are major CYP2D isoforms expressed in the rat. In humans, only CYP2D6 is expressed. In rat brain, the mRNA for CYP2D4 is most abundant in cerebellum, striatum, pons and medulla oblongata. In human brain, CYP2D6 mRNA expression was detected in all regions with highest levels observed in cerebellum. CYP2D isoforms are involved in the metabolism of not only xenobiotics such as antidepressants, beta-adrenergic blockers, antiarrhysthmics, and antihypertensives, but also endogenous compounds such as trace amine and neurosteroids. Among 11 isoforms of human recombinant P450s, only CYP2D6 exhibited an ability to efficiently convert tyramine which exists in the brain, to dopamine. CYP2D4 and CYP2D6 which are the predominant CYP2D isoforms in the rat and human brain, respectively, possess 21-hydroxylation activity for both progesterone and allopregnanolone. CYP2D4, not P450c21, works as a steroid 21-hydroxylase in the brain. These results suggested that CYP2D in the brain may be involved in the metabolism of neuronal amines and steroids and in the regulation of the central nervous system.