Nephrotoxic nephritis was induced in Sprague-Dawley and Munich-Wistar rats by the injection of rabbit antirat kidney serum. A biphasic pattern of proteinuria was induced: the heterologous phase with a peak of proteinuria occurring at 10 to 16 hours, and the autologous phase with a peak at 10 to 15 days. For morphologic studies, glomeruli were fixed by perfusion, or by drip-fixation during good blood flow. In the heterologous phase, glomerular endothelial detachment or loss and leukocytic infiltration were prominent. In the autologous phase, focal detachment of glomerular endothelium and epithelium was commonly found. At sites of endothelial loss, in both phases, endogenous albumin (demonstrated by an ultrastructural immunoperoxidase technique), but not intravenously injected ferritin, showed abnormally deep penetration into the glomerular basement membrane. At sites of epithelial loss, found in the autologous phase, both albumin and ferritin were detected throughout the glomerular basement membrane. It is proposed that, in glomerular disease, leakage of plasma proteins may occur across the glomerular basement membrane at sites of endothelial or epithelial detachment.