Using differential display methodology, we isolated a tamoxifen-regulated cDNA. This cDNA was identical to the ps20 cDNA isolated from urogenital sinus mesenchymal cells. ps20 expression was detected in various female rat tissues, with the highest expression in lung and heart. ps20 transcripts were low during estrus and proestrus, but high during the diestrous stage of the estrous cycle coincident with estrogen-induced uterine cell proliferation. Treatment of ovary-intact or ovariectomized rats with estrogens or tamoxifen resulted in increased uterine weight and decreased ps20 expression. Uterine involution associated with ovariectomy or antiestrogen treatment led to up-regulation of ps20. Antibody against rat ps20 recognized the native rat ps20 in conditioned medium of primary rat uterine cells and stable ps20-transfected MCF-7 cells with molecular masses of approximately 24, 27, and 29 kDa. In primary rat uterine cells, ps20 secretion was enhanced by ICI 182,780, but was inhibited by estrogens and tamoxifen. Immunohistochemistry revealed that ps20 was localized to smooth muscle and luminal epithelial cells as well as the glandular population of uterine tissue. Conditioned medium derived from ps20-transfected MCF-7 cells, but not Escherichia coli recombinant ps20, exhibited mild growth suppression on PC-3 cells. The data indicate that ps20 expression is negatively regulated by estrogens and tamoxifen and suggest that ps20 may function as a mediator of local growth.