Chronic focal glomerulonephritis was induced in the rat by using goat nephrotoxic IgG. In the later stage (6 to 15 months) a number of glomeruli were found to be attached to Bowman's capsule with variable segmental sclerosis and/or hyalinosis. The morphogenesis of the changes was studied by electron microscopy. It was found that partial cytoplasmic necrosis of podocytes was an initiating event of the capsular adhesion. When this occurred in the peripheral tufts, the denuded portion of the glomerular basement membrane (GBM) became attached to the parietal epithelium, resulting in progressive thickening of the mesangium and GBM, deposition of hyaline or proteinaceous material, and desquamation of the endothelial sheet, the features being compatible with segmental glomerular sclerosis and/or hyalinosis. It seemed that these changes, in turn, accelerated podocyte desquamation and extension of the capsular adhesion, producing a progressive cycle that terminated in global glomerular hyalinosis. Such a sequence of events most often occurred in rats with massive proteinuria, suggesting that persistent, severe proteinuria caused the podocyte lesion which led to the development of segmental glomerular sclerosis and/or hyalinosis.