Despite recent discoveries of molecules in podocytes, the mechanisms behind most conditions of proteinuria are still poorly understood. In order to understand more about this delicate barrier, we studied the functional and morphological effects of mild (15 min) renal ischemia reperfusion injury, IRI. Renal function was studied in rats in vivo, followed by a more detailed analysis of the glomerular barrier in cooled (8 °C) isolated perfused kidneys (cIPK). Renal blood flow was quickly restored, whereas GFR remained halved 30 min after IRI. Tubular cell activity was intact as judged from the unaffected Cr-EDTA U/P concentration ratio. In vivo, the fractional clearance () for albumin increased 16 times. In rats subjected to cIPK starting 30 min after in vivo IRI, _albumin was 15 times and _{Ficoll_36A} 1.8 times higher than in control cIPKs. According to the heterogeneous charged fiber model, IRI reduced the fiber charge density to 38% of control (P<0.01, N=7). Morphometric analysis with electron microscopy did not reveal any changes of the podocytes or the glomerular basement membrane (GBM) after IRI, suggesting more subtle changes of the GBM and/or the endothelial glycocalyx. We conclude that mild renal IRI induce formation of reactive oxygen species, massive proteinuria, and loss of charged fibers with no apparent change in morphology. These novel findings stress the importance of other components of the barrier such as proteoglycans produced by the glomerular cells and provide a tentative explanation for the mechanisms behind proteinuria in e.g. glomerulonephritis.