The presence and activity of cytochromes P450 in brain regions and various brain cells have been extended and advanced over the last five years covered by this review. Using in situ hybridization and immunohistochemical techniques, many cytochrome P450 enzymes have been demonstrated to be present in brain and to have a regional rather than universal distribution. Many of these various cytochromes P450 have been shown to catalyze the metabolism of neurosteroids as well as other biologically significant compounds in brain. In addition, many cytochrome P450 enzymes have been implicated in the metabolism of psychoactive drugs such as neuroleptics and antidepressants. The regulation of cytochrome P450 expression has been studied at greater detail, the regulation of aromatase being a prominent example during the last five years.