The mammalian pro- and mesonephros are transient embryonic kidneys essential for urogenital system development. The nephric (Wolffian) duct,which is a central constituent of both structures, elongates caudally along a stereotypical path to reach the hindlimb level where it induces metanephros(adult kidney) formation, while the remaining duct gives rise to the male genital tract (epidydimis, vas deferens). The transcription factors Pax2 and Pax8 are essential for the initiation of pro- and mesonephros development. In a cDNA microarray screen for genes specifically expressed in the pro/mesonephros and regulated by Pax proteins, we identified Gata3, a transcription factor gene associated with hypoparathyroidism, deafness and renal anomaly (HDR) syndrome. Gata3 is already expressed in the pronephric anlage, together with Pax2 and Pax8, suggesting that it may be a direct Pax2/8 target gene. Inactivation of Gata3 by insertion of an Ires-GFP reporter gene resulted in a massive increase in nephric duct cellularity, which was accompanied by enhanced cell proliferation and aberrant elongation of the nephric duct. Interestingly,however, the nephrogenic cord extended, with delayed kinetics, along the entire caudal path up to the level of the hindlimb bud, indicating that extension of the nephric duct and cord is controlled by different guidance cues. At the molecular level, the nephric duct of Gata3-/-embryos is characterized by the loss of Ret expression and signaling, which may contribute to the guidance defect of the nephric duct. Together, these results define Gata3 as a key regulator of nephric duct morphogenesis and guidance in the pro/mesonephric kidney.