Dipeptidylpeptidase IV (DPPIV) is an exopeptidase highly expressed in the brush-border membrane of the small intestine and in the proximal renal tubules. In this paper the 5'-flanking region of the DPPIV gene containing the promoter was sequenced and functionally characterized. The porcine DPPIV promoter lacks a consensus TATA-box, but contains two TATA-like sequences. Evidence for multiple transcription initiation sites was found. Different deletion constructs of the DPPIV 5'-flanking region in front of the CAT gene were analyzed for transient CAT-expression after transfection of the intestinal Caco-2 cell line. These experiments showed that a 89 base pair construct (-91 to -3 relative to the translation initiation site) is sufficient for promoter activity. In the reverse orientation this construct also stimulates transcription with a similar effectivity indicating that the DPPIV promoter has a bidirectional function. The bidirectional function was further demonstrated by the introduction of the -91 to -3 construct into the bidirectional vector system pLUC/CAT-3. In the hepatoma cell line HepG2 two selected deletion constructs (-857 to -3; -282 to -3) were analyzed in the normal orientation using the CAT gene as a reporter gene. The transfection experiments showed that deletion of the region -857 to -282 raised the promoter activity 3-fold. The GC-rich 5'-flanking region was further analyzed and we demonstrate that the DPPIV promoter contains a region with the characteristics of an unmethylated CpG island.