We analyzed matrix metalloproteinase (MMP) production by 11-d embryonic mouse kidneys and the effects of these enzymes on subsequent renal organogenesis. In vivo, immunolocalization of metalloproteinases by laser scanning confocal microscopy and zymograms of kidney lysates showed that the mesenchyme of embryonic kidneys synthesized both MMP9 and MMP2 enzymes. In vitro, embryonic kidneys also secreted both enzymes when cultured in a medium devoid of hormone, growth factor, and serum for 24 h during which T-shaped branching of the ureter bud appeared. We then evaluated the role of MMP2 and MMP9 in kidney morphogenesis by adding anti-MMP2 or anti-MMP9 IgGs to the culture medium of 11-d kidneys for 24 or 72 h. Although it inhibited activity of the mouse enzyme, anti-MMP2 IgGs had no effect on kidney morphogenesis. In contrast, anti-MMP9 IgGs with enzyme-blocking activity impaired renal morphogenesis, in a concentration-dependent manner, by inhibiting T-shaped branching and further divisions of the ureter bud. This effect was irreversible, still observed after inductive events and reproduced by exogenous tissue inhibitor of metalloproteinase 1 (TIMP1), the natural inhibitor of MMP9. These data provide the first demonstration of MMP9 and MMP2 production in vivo by 11-d embryonic kidneys and further show that MMP9 is required in vitro for branching morphogenesis of the ureter bud.