Compelling evidence exists displaying that the intrafollicular IGF-I system constitutes an obligatory mediator of FSH action in the murine ovary. Within this system, the ovarian IGF binding protein-4-directed protease (IGFBP-4ase) may have a critical role. Human IGFBP-4ase has been proved identical to the previously well-characterized pregnancy-associated plasma protein-A (PAPP-A). This communication reports the cloning and sequencing of the mouse PAPP-A cDNA as well as its expression and cellular localization in the mouse ovary. PAPP-A mRNA was undetectable in ovaries of untreated immature 25-d-old mice. Treatment with PMSG led to a marked time-dependent increase in PAPP-A expression in well-defined subsets of granulosa cells and follicles. Specifically, PAPP-A expression was detectable exclusively in centrifugally residing membrana granulosa cells of antral follicles during a 3- to 36-h period post PMSG. PAPP-A expression then fell to nondetectable levels in dominant preovulatory follicles at 48 h post PMSG. Treatment of PMSG-primed mice with human CG caused a rapid reinduction of PAPP-A expression in granulosa cells of dominant follicles and was sustained at relatively high levels throughout the ovulation and luteinization. These results suggest a role for gonadotropin-stimulated PAPP-A gene expression in the physiologic processes of dominant follicle development, ovulation, and luteogenesis in the mammalian ovary. The early onset and extended duration of gonadotropin-dependent PAPP-A expression in granulosa cells may serve to degrade the antigonadotropin IGFBP-4. Accordingly, successful antral follicle development, ovulation, and corpus luteum formation may be contingent on an IGFBP-4-deplete/PAPP-A-replete circumstance, hence resulting in an IGF-I-replete intrafollicular microenvironment.