Inducible hormone-dependent tryptophan oxygenase gene is expressed mostly in the liver under the control of glucocorticoid hormones. In the regulatory region of this gene there are three constitutive sites independent of hormone presence and gene expression. While investigating transcription factors responsible for the formation of specific chromatin structure in the regulatory region of inducible genes it is necessary to identify proteins which can bind to DNA specifically in these sites. The present paper reports investigation of transcription factors which bind to DNA from the -292nd to the -178th nucleotide of gene to in vitro. This DNA region contains a site corresponding to the constitutive DNase I hypersensitive site in vivo. Using electrophoretic mobility shift assay, we have analysed binding of rat liver proteins to this DNA region. NF1-rich nuclear protein fraction was purified from the rat liver nuclear extract by DEAE-cellulose and heparin-sepharose chromatography. Studies of the competition with a consensus sequence site for NF1 recognition sites have shown that it is the NF1 family transcription factors that are responsible for the formation of these specific complexes.