Plasminogen activator inhibitor-1 (PAI-1) is a specific and rapid inhibitor of tissue plasminogen activator (tPA) and urokinase. Clinical studies suggest that PAI-1 may play a crucial role in the regulation of fibrinolysis. A number of factors modulate PAI-1 activity in endothelial cell culture, and the isolation of PAI-1 cDNA now allows study of PAI-1 regulation at the mRNA level. We examined the effect of endothelial cell growth factor (ECGF) and heparin on PAI-1 expression in human umbilical vein endothelial cell (HUVEC) culture. The addition of ECGF and heparin to HUVEC cultures results in a 3-10-fold decrease in the PAI-1 activity secreted into the conditioned media. This effect is mediated at the mRNA level. A decrease in PAI-1 is also seen with higher concentrations of ECGF alone, but is greatly enhanced by the addition of heparin. No significant change in tPA antigen or mRNA levels was observed.