The nucleotide sequence of the mRNA for NADPH-cytochrome P-450 reductase from rabbit liver was determined from a full-length cDNA clone (pFP105). The clone contains 2,269 nucleotides complementary to rabbit liver reductase mRNA. The single open reading frame of 2,037 nucleotides codes for a 679-amino acid polypeptide with a calculated molecular weight of 76,583 daltons. The cloned cDNA contains the complete 3'-noncoding region of 193 nucleotides, including 68 nucleotides of poly(A), and 39 nucleotides of the 5'-noncoding region. The nucleotide sequence in the coding region of cDNA of rabbit reductase (pFP105) showed 85% homology to that of rat reductase (Porter, T.D. & Kasper, C.B. (1985) Proc. Natl. Acad. Sci. U.S. 82, 973-977, and Murakami, H. et al. (1986) DNA 5, 1-10). Rabbit reductase has one more amino acid residue than the rat enzyme, and the amino acid compositions of the two enzymes are similar. The amino acid sequence of the rabbit enzyme showed 91% identity with that of the rat enzyme. The segment related to binding of FMN and FAD was well conserved among rabbit, rat, and pig reductases. The sequence related to AMP moiety-binding was also conserved among these species, and was found in the amino acid sequence of NADH-cytochrome b5 reductase, another flavoenzyme in the microsomal electron transport system.