The hardening reaction that occurs in the zona pellucida to block polyspermy can be overcome in oocyte cultures in the presence of fetal serum or the serum component fetuin. Fetuin may also prevent precocious zona hardening by inhibiting a ZP2 proteinase released spontaneously by cortical granules during maturation of the oocyte. We demonstrated fetuin mRNA in the rat ovary by reverse transcriptase-polymerase chain reaction and localized it by in situ hybridization. Fetuin mRNA was present in all granulosa cells of growing and large follicles. Immunohistochemical analysis revealed that the fetuin protein was only present in some of the small, growing follicles. In large, healthy follicles, fetuin protein was confined to cumulus cells and granulosa cells bordering the antrum. Fetuin was present in atretic follicles, but the staining pattern differed from that of healthy follicles. The follicular antrum contained a substantial amount of fetuin, but whether granulosa cells secreted it or it originated in the ovarian blood supply could not be confirmed. We concluded that at least a portion of the fetuin is produced by granulosa cells of growing and large follicles, suggesting that fetuin may function in a paracrine manner to maintain the zona pellucida in a penetrable state for fertilization.