Cytochrome P450-catalyzed monooxygenation system is widespread in nature and many isoforms have been found in almost all the tissues of vertebrates. In the kidney, several isoforms of cytochrome P450 families, CYP1, 2, 3, 4, 24, 27 have been characterized by biological, immunochemical and molecular biological techniques. These cytochrome P450 isoforms are localized in the endoplasmic reticulum or mitochondria of kidney cortex and functional in the monooxygenation of a number of substrates, such as steroids, fatty acids, xenobiotics, and vitamin D. Particularly, hydroxylation of vitamin D at positions of 1 and 24 is known to be one of the most important functions of kidney specific P450 isoforms.