Multiple cis-acting DNA sequences regulating expression of the rat liver NADPH-cytochrome P-450 oxidoreductase gene have been identified in transient transfection assays using promoter deletion constructs linked to the chloramphenicol acetyl transferase gene. The TATA-less promoter possesses nine GC-boxes which contain the consensus sequence for the transcription factor Sp1. While loss of the seven distal GC-boxes had minimal effect on transcriptional activity, deletion of the next 35 bp, from -206 to -172, resulted in approximately 90% loss of promoter activity. Contained within this region is an Sp1 binding site indicating that either (1) this particular consensus sequence was essential for transcription, (2) the two proximal GC boxes act in concert, or (3) a yet unidentified regulatory element resides within this 35-bp stretch. In addition, transfection experiments demonstrated that two separate distal regions (-622 to -1167 and -1500 to -2300) contain negative regulatory elements which down-regulate gene transcription in a position-independent manner. Mobility-shift analyses and DNase footprinting identified sequences in the proximal region of the promoter that bound proteins present in nuclear extracts. Four protected segments were observed within the first 100 bp upstream of the transcription start site; these include (1) the region encompassing the transcription start site (-7 to +4), (2) the region normally occupied by a TATA-box (-38 to -18), (3) the bases from -78 to -60 which contain the regulatory element CACC, and (4) bases -105 to -92 which include an Sp1 binding site. Hence, regulation of the NADPH-cytochrome P-450 oxidoreductase gene is controlled by both positive and negative regulatory elements, and, of the nine Sp1 consensus sites, the two proximal sites are sufficient to support basal transcription.