||The steroid hormone estrogen is a critical regulator of sexual development and fertility in the female. An understanding of the molecular mechanisms of estrogen action is therefore essential for management of human reproductive health and fertility. The ovariectomized (OVX) immature rat as a model system disruption of the highly dense fibrillar collagen network of the extracellular matrix occurs rapidly in the early hypertrophic growth phase leading to a near complete dissolution of organized matrix within 4 hours. The mechanism by which it occurs is poorly understood. ECM remodeling likely reflects E 2 -mediated changes in activity of matrix metalloproteinases (MMPs), ubiquitous matrix degradative enzymes which require activation by cleavage of the propeptide domain. MMP substrates include all known components of the ECM, therefore regulation of MMP activity is crucial to fine tune matrix composition and structure. Characterization of the mechanism linking estrogen signaling to modulation of MMP activity will provide insight as to the role of the hormone in regulation of uterine tissue remodeling that is critical for fertility. One upstream candidate that can activate MMPs through propeptide cleavage is the enzyme plasmin. Plg must be proteolytically processed to produce active plasmin. Activating components include tissue Plg activator (tPA) and urokinase Plg activator (uPA) which catalyze conversion of Plg to plasmin. Inhibitory components include plasmin inhibitors (PAI-1 and 2) which bind to plasmin activators and inactivate them. My research focused on E 2 -induced changes in expression of PA system components and analysis of uterine plasmin production as an upstream using quantitative Western blot and fluorescent in situ zymography to localize areas of plasmin production within hormone treated tissues via light microscopy. It was found that an increase in both plasminogen plasmin uPA, tPA, PAI-1 occur within 4 hours of E 2 treatment a time point that corresponds to the peak decrease in matrix collagen density. Pretreatment with the steroid anti-inflammatory dexamethasone (Dex) inhibits E 2 -induced upregulation of uPA, tPA, and PAI-1 but not plasminogen or plasmin. These data do confirm that Dex has a repressive effect on the PA system. Analysis of plasmin activity via in situ zymography indicates an increase in active enzyme in response to E 2 but complete time course analysis was not realized. Taken together, these data demonstrate a potential role for the PA system in the ECM remodeling of the rat uterus.