In the human fetal liver, CYP3A7 is expressed as early as the 13th week of gestation. This continues to the perinatal period when it is sharply repressed prior to birth. Concomitantly, the expression of CYP3A4, not detectable in the fetus, sharply increases in the perinatal period to remain elevated throughout adulthood. The mechanisms controlling these developmental patterns of expression have not yet been elucidated at the molecular level. The aim of the present work was to make a functional analysis of the 5'-flanking regions of CYP3A4 and CYP3A7 in different cell lines, including CHO, HepG2, WRL68 and Caco-2 TC7, after cotransfection with two hepato-specific transcription factors, C/EBP alpha and DBP. Six deletions of different length of the 5'-flanking region of each gene, spanning from -1240 to +11 for CYP3A4 and from -1157 to +13 for CYP3A7, were analysed by reporter gene assay. With the CYP3A4 constructs, C/EBP alpha stimulated the transcriptional activity in CHO cells in a way that suggested the presence of at least two C/EBP alpha-responsive elements, one downstream of -55 and one upstream of this position. In CYP3A7, the proximal element exhibited comparable stimulation to the corresponding one in CYP3A4, although the more distal one appeared to respond to a much smaller extent. CYP3A4 and CYP3A7 constructs also responded to C/EBP alpha in HepG2 and WRL68. However, only CYP3A4 and not CYP3A7 was transactivated by this factor in the Caco-2-TC7 cell line. In CHO cells, only the shortest proximal promoter deletion of CYP3A4 (downstream of -57) responded to DBP, while neither the longer constructs nor the CYP3A7 deletions were transactivated. Although preliminary, our results suggest that C/EBP alpha, and possibly other members of the C/EBP family, play a prominent part in the expression of the CYP3A family in man, and that the two genes respond differently to C/EBP alpha and DBP, two factors that exhibit a strict proliferation-dependent pattern of expression in the liver.