Dynamics of nuclear translocation of transcription factor Stat1 in human epidermoid carcinoma A431 cells in response to the epidermal growth factor (EGF) was examined by immunofluorescent microscopy and in cytoplasmic and nuclear extracts by Western blot. In has been shown that a prolonged presence of EGF induces a rapid tyrosine phosphorylation and nuclear translocation of Stat1 (within 5 min). The maximum amount of this protein in the nucleus was reached 30 min after the cell treatment to be maintained at the same level for 5 h. To study the dynamics of the export of Stat1 from the nucleus, a gentle treatment of cells with acetate buffer, pH 4.5, was used for extracting the surface-bound EGF. In this case, a complete dephosphorylation of Stat1 in the cytoplasm was observed in 30 min and the export of the Stat1 from the nucleus lasted for 1-6 h. These studies suggest the existence of an EGF-dependent dynamic equilibrium between the import and export of Stat1 in A431 cells.