A cytochrome P-450 N (N for Neonatal) was resolved and partially purified from 15-day-old newborn rat livers. Antibody raised in rabbit against P-450 N detects two bands in liver microsomes, the major one comigrating with the partially purified P-450 N. This protein is not expressed in lung and kidney and exhibits an unusual ontogenic profile in the liver with maximal levels in 2-week-old animals. Expression of P-450 N is not modified in animals given beta-naphthoflavone or pregnenolone 16 alpha-carbonitrile, but is markedly increased by phenobarbital. Its amino-acid composition differs from that of other P-450s and it is not recognized by antibodies directed against CYP1A, CYP2A1, CYP2B, CYP2C11, CYP3A or epoxide hydrolase. It could thus be considered as an unique P-450 expressed transiently before puberty.