A novel P450 cDNA, designated IVA3, was isolated from a lambda gt11 cDNA library from clofibrate-treated rat liver by screening with the lauric acid omega-hydroxylase, IVA1, cDNA probe. This cDNA encoded a protein with 507 amino acids and a calculated Mr of 58,239. The IVA3 cDNA shared 65% and 97% nucleotide and 72% and 96% DNA-deduced amino acid sequence similarities with IVA1 and IVA2, respectively. The CYP4A gene family, designated the Cyp4a locus, was mapped to mouse chromosome 4 using a panel of mouse-hamster somatic cell hybrids. Levels of the IVA mRNAs were analyzed in rat tissues and cell cultures after treatment with the hypolipidemic drug clofibrate. The IVA1, IVA2, and IVA3 mRNAs were present at very low levels in the livers of untreated rats and markedly induced in rats treated with clofibrate. Dose-response and time-course studies revealed that all three genes were regulated coordinately in liver. In contrast to the coordinate induction in liver, only the CYP4A3 gene was induced in the rat hepatoma cell line McA-RH7777. In the kidney, IVA1 and IVA3 mRNAs were present at low levels and were induced by clofibrate in a manner similar to that in liver. In contrast, the IVA2 mRNA was expressed in the kidney of untreated rats at a level similar to that of the maximally induced IVA2 mRNA in liver. These data indicate that the CYP4A1, CYP4A2, and CYP4A3 genes are regulated coordinately in liver while only CYP4A2 is activated constitutively in kidney.