1 The renal medulla is a major source of plasminogen activators (PA), recently shown to induce vasodilation in vitro. Treatment with PA inhibitors has been associated with renal dysfunction, suggesting compromised renal microvasculature. We investigated the impact of the PA inhibitor epsilon amino-caproic acid (EACA) upon vascular tone in vitro, and studied the effect of both tPA and EACA upon intrarenal hemodynamics in vivo. 2 In vitro experiments were carried out in isolated aortic rings and with cultured vascular smooth muscle cells. Studies of renal microcirculation and morphology were conducted in anesthetized Sprague-Dawley rats. 3 In isolated aortic rings, EACA (but not the other inhibitors of the fibrinolytic system PAI-1 or a-2 antiplasmin) reduced the half-maximal effective concentration of phenylephrine (PE) required to induce contraction (from 32 nM in control solution to 2 and 0.1 nM at EACA concentrations of 1 and 10 mM, respectively). Using reteplase (retavase) in the same model, we also provide evidence that the vasoactivity of tPA is in part kringle-dependent. In cultured vascular smooth muscle cells, Ca 2 þ internalization following PE was enhanced by EACA, and retarded by tPA. 4 In anesthetized rats, EACA (150 mg kg À1 ) did not affect systemic blood pressure, total renal or cortical blood flow. However, the outer medullary blood flow declined 1272% below the baseline (Po0.03). By contrast, tPA (2 mg kg À1 ), transiently increased outer medullary blood flow by 875% (Po0.02). Fibrin microthrombi were not found within the renal microvasculature in EACA-treated animals. 5 In conclusion, both fibrinolytic and antifibrinolytic agents modulate medullary renal blood flow with reciprocal effects of vasodilation (PA) and vasoconstriction (EACA). In vitro studies suggest that these hemodynamic responses are related to direct modulation of the vascular tone.