The plasminogen activator inhibitor-1 (PAI-1) expression is induced by mild hypoxia (8 % O2) via the PAI-1 promoter region -175/-159 containing a hypoxia response element (HRE-2) binding the hypoxia inducible factor-1 (HIF-1) and an adjacent response element (HRE-1) binding a so far unknown factor. The aim of the present study was to identify this factor and to investigate its role in the regulation of PAI-1 expression. It was found by supershift assays that the upstream stimulatory factor-2a (USF-2a) is bound mainly to HRE-1 of the PAI-1 promoter and to a lesser extent to HRE-2. The USF-2a overexpression inhibited both PAI-1 mRNA and protein expression in primary cultured rat hepatocytes under normoxia and hypoxia. The Luciferase (Luc) reporter gene constructions driven by 766 and 276 bp of the 5'-flanking region of the rat PAI-1 gene were transiently transfected into primary rat hepatocytes together with the expression vectors encoding a wild type USF-2a and a USF-2a mutant lacking DNA-binding and dimerization activity (ΔHU2a). The cotransfection of the wild type USF-2a vector reduced the Luc activity by about 8 fold, whereas the cotransfection of ΔHU2a did not influence this activity. Mutation of the HRE-1 (-175/-168) in the rat PAI-1 promoter Luc constractions decreased the VSF-dependent inhibition of the Luc activity. Mutation of the HRE-2 (-165/-158) was less effective. These results indicated that the balance between two transcriptional factors, HIF-I and USF-2a, which could bind adjacent the HRE sites, appeared to be involved in the regulation of the PAI-1 expression under many clinical conditions.