Two closely related cDNAs encoding carbonyl reductase (CR) were isolated from the rat ovary and testis. One (inducible CR, iCR) was rapidly and strongly induced in the ovary by pregnant mare serum gonadotropin (PMSG), whereas the other (non-inducible CR, nCR) was constitutively expressed in the ovary and was not induced by PMSG. Both genes were also expressed in the rat testis. The cDNAs of rat iCR and nCR encode highly homologous proteins (86% identity in amino acid) with 277 and 276 amino acid residues, respectively. However, the 5'-upstream regions flanking respective genes are completely unrelated to each other except for a short (38 bp) overlap. These results indicate the presence of two closely related but differently regulated CR genes in rat gonadal tissues. Strong induction of iCR by PMSG suggests the importance of the gene in the ovarian follicular development.