The dependence of urokinase (uPA) secretion on basic fibroblast growth factor (bFGF) and platelet-deprived growth factor (PDGF BB) was investigated by using of cultured rat aortic smooth muscle cells (SMC). Growth factors stimulated secretion of urokinase with two-phase kinetics within 1-60 minutes. Namely, 'apparent' concentration of uPA in the conditioned media rised to 12.5 nM within the first 1-2 min and 5.8 nM after 30 min of cell stimulation by growth factors, and decreased to basal level at 5 min after stimulation of the cells. The character of uPA-secretion kinetics was similar in response to both growth factors, but the level of secreted uPA was higher in case of PDGF BB. We have shown that this decrease of uPA content in conditioned media is not related to the binding of uPA to the cell surface receptors or extracellular matrix proteins. One can suppose that urokinase secreted within 5 minutes could bind to secretory protein which nature has to be identified. But it was established that this secretory protein, complexing urokinase in cultured media, is not identical to the plasminogen activator inhibitor type 1 (PAI-1).