We recently established and characterized two rat endometrial adenocarcinoma cell lines which we called RUCA-I and RUCA-II. Despite high estrogen receptor levels neither cell line responded to estradiol in conventional cell culture conditions on plastic and in the presence of charcoal stripped fetal calf serum. We further demonstrated that culturing of these cells on a reconstituted basement membrane induced the estrogen responsiveness for both proliferation and gene expression. Particularly, the expression of components of the complement C3 system, which represent major estradiol inducible proteins in the rat uterus in vivo, were found to be under the control of estrogens and antiestrogens. In this paper the search for estrogen repressed proteins is reported. For this purpose secretory proteins of RUCA-I cells were metabolically labelled with 35S-methionine and tested for the presence of estrogen-repressed, antiestrogen-inducible protein species. Analyzing cell culture supernatants of RUCA-I cells by polyacrylamide gel electrophoresis under reducing conditions a protein with an apparent size of approx. 250-270 kDa became conspicuous. The formation and secretion of this protein was suppressed by estradiol and induced by the antiestrogen ICI 164384. Gel electrophoresis performed under non-reducing conditions and hyaluronidase digestion showed that this estrogen-repressed protein represents a dimeric glycoprotein. By immunoprecipitation this glycoprotein was identified as fibronectin. Investigations of steady state mRNA levels of fibronectin by rtPCR suggested a post-transcriptional regulation of this molecule by estradiol. This is the first report on repression of components of the extracellular matrix by estradiol and induction by the complete antiestrogen ICI 164384. The consequences of this finding in regard to growth and invasion of endometrial tumors are discussed.