The Pax family of transcription factors plays important roles in vertebrate organogenesis. Pax-2 is a critical factor in the development of the mammalian urogenital system. Pax-2 is expressed in the epithelia of the ureter, the Müllerian duct, and the Wolffian duct and in the nephrogenic mesenchyme. Gene targeting in the mouse as well as natural mutations in mouse and man have demonstrated the requirement of Pax-2 in the development of these structures. Little is known about the molecular mechanisms regulating Pax-2 expression in the developing urogenital system. As a first step to reveal these mechanisms and to search for the elements and factors controlling Pax-2 expression we have characterized regulatory sequences of the Pax-2 gene in an in vivo reporter assay in the mouse. An 8.5-kb genomic region upstream of the Pax-2 transcription start site directed reporter gene activity in the epithelium of the pronephric duct at 8.25 days postcoitum (dpc) and in the Wolffian duct starting from 9.0 dpc. Expression in the Wolffian duct and its derivatives, the ureter, the collecting duct system, the seminal vesicles, the vas deferens, and the epididymis, was maintained at least until 18.5 dpc. Hence, an element(s) in the 8.5-kb upstream region is sufficient to initiate and maintain Pax-2 expression in the Wolffian duct and its derivatives. In order to more precisely map the Wolffian duct regulatory sequences, a deletion analysis of the 8.5-kb upstream region was performed in a transient in vivo reporter assay. A 0.4-kb subfragment was required for marker gene expression in the Wolffian duct. Misexpression of fgf8 under the control of the 8.5-kb upstream region resulted in polycystic kidneys, demonstrating the general usefulness of Pax-2 regulatory sequences in misexpression of foreign genes in the ureter and collecting duct system of the kidney in transgenic approaches in mice.