Intrarenal distribution of cytochrome P-450 (P-450) was investigated with different segments of isolated single nephrons from rabbits and rats by using a new ultra-micro method of P-450 determination. In both animals, P-450 was localized only in the proximal tubule. Other segments such as the glomerulus, the loop of Henle, the distal tubule, and the collecting tubule possessed no P-450 at all. Within the proximal tubule, the straight portion (S2 and/or S3 segments) revealed a higher specific content of P-450 than the convoluted one. An inducer of P-450, 3, 4-benzo(a)pyrene increased the P-450 of a definite portion (second 3 mm from the glomerulus) almost doubly in rabbits. In rats, both 3-methylcholanthrene (3MC) and 48 hr starvation (Fast) induced P-450, but only the later increased laurate-omega-hydroxylation. P-450 in the proximal convoluted and straight tubules was induced separately by Fast and 3MC, respectively. These results indicate that renal mixed function oxidase should be confined to the proximal tubule and that, like the liver, multiple forms of renal P-450 should exist in the different proximal segments.