The purpose of this study was to identify cyclic variations and hormonal regulation of endometrial transforming growth factor beta1 (TGFbeta1) mRNA. Regulation of the plasminogen-activating system was also examined, since it is involved in activation of latent TGFbetas. We measured TGFbeta1 mRNA in 51 normal endometrial samples by Northern blot and densitometric scanning of autoradiograms. Each value was related to the corresponding beta-actin value to allow quantitative evaluation. TGFbeta1 mRNA was higher in the mid and late secretory and menstrual phases than in the earlier parts of the cycle. This pattern implies progesterone dependence. The content of TGFbeta1 mRNA in endometrial tissue explants obtained in the proliferative phase was significantly increased after stimulation for 4 days with estradiol + progesterone in vitro. Both TGFbeta1 and estradiol + progesterone increased the content of plasminogen activator inhibitor-1 mRNA and protein in primary cultures of endometrial stromal cells. Conditioned-medium concentrations of urokinase plasminogen activator (u-PA) were increased by TGFbeta1, but decreased by estradiol + progesterone. This effect of estradiol + progesterone results from increased internalization and degradation of u-PA secondary to up-regulation of the cell surface receptor for u-PA by progesterone (Casslén et al., JCEM 1995; 80: 2776-2784). Increased extracellular u-PA in response to TGFbeta1 exposure was thus in concordance with an unchanged amount of available u-PA receptors on the cell surface. The activation mechanism of latent TGFbeta involves u-PA activity; since u-PA activity is reduced in the secretory endometrium, we suggest that although TGFbeta1 mRNA is increased in the mid and late secretory phase, TGFbetas are mainly in their latent form until the premenstrual rise in u-PA activity stimulates activation. TGFbeta may promote capillary growth during endometrial regeneration.