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Cystatin C is a secreted inhibitor of cysteine proteinases that participates in extracellular matrix remodeling. Whether hormones affect its expression in the vagina was unknown. Consequently, we examined the effects of estradiol (E(2)), progesterone (P), and raloxifene on vaginal cystatin C in rhesus macaques. In experiment 1, ovariectomized animals were treated sequentially with E(2) (14 d) and E(2) + P (14 d) to induce 28-d menstrual cycles. Vaginal samples were collected on d 6, 8, 14, and 28 of the induced cycle. Some cycled animals were deprived of both E(2) + P for 28 d. In experiment 2, ovariectomized animals were treated for 5 months with E(2) alone, E(2) + P, raloxifene, or left untreated. Total RNA from the vaginal wall was analyzed for the cystatin C transcript with a commercially prepared cDNA array and semiquantitative RT-PCR. Vaginal cryosections were analyzed by in situ hybridization for cystatin C transcript and by immunocytochemistry for the protein. E(2) treatment significantly (5-fold; P < 0.05) increased expression of cystatin C transcript over the levels in the hormone-deprived controls, and cotreatment with P (E(2) + P) blocked this effect. Raloxifene treatment did not affect cystatin C expression. In situ hybridization and immunocytochemistry revealed that cystatin C was localized in fibroblasts and smooth muscle cells throughout the vaginal wall but not in smooth muscle cells of arteries or levator ani myocytes. In summary, E(2) increased vaginal cystatin C expression in the fibroblasts and smooth muscle bundles, P suppressed this effect, and raloxifene had no effects on cystatin C. Elevated cystatin C, by suppressing cysteine proteinase activity, may strengthen the vaginal wall and mitigate the potential for pelvic floor prolapse.