A cDNA for rat prolyl oligopeptidase was cloned which contained an open reading frame of 2,130 nucleotides encoding a protein of 710 amino acids. The deduced amino acid sequence is around 95% homologous to other mammalian prolyl oligopeptidases and about 40% to bacterial prolyl oligopeptidases. The recombinant prolyl oligopeptidase generated in E. coli was purified and its properties were examined. The substrate specificity and the susceptibility to proteinase inhibitors were similar to those of the native enzyme. Northern blot analysis showed wide expression of the prolyl oligopeptidase gene. Using ovaries from hormone-treated rats, it was found that both the mRNA expression and enzyme activity increased in the luteal phase. These findings suggest the involvement of prolyl oligopeptidase in events associated with corpus luteum formation and/or luteal regression.