Rats progressively develop proteinuria and glomerular sclerosis with age. These physiologic and histologic changes are accelerated by subtotal renal mass ablation. Alterations in the glomerular basement membrane (GBM), such as thickening and decreased anionic site density, occur during senescence. This study examines the ultrastructural alterations of the GBM affecting remnant glomeruli. Six week-old male Wistar rats underwent a subtotal nephrectomy (70%) and were compared with sham operated rats. Rats were fed a standard diet, and physiologic measurements were performed every 3 weeks. Rats were killed 6 and 12 weeks after surgery. Anionic sites were labeled by polyethyleneimine perfusion before killing. Kidneys were processed for light and electron microscopy. Nephrectomized rats had higher protein-uria than the controls and developed glomerular sclerosis. The GBM of nephrectomized rats were significantly thinner and anionic sites were less numerous 12 weeks after nephrectomy, especially in the lamina densa (LD) of the GBM. The number and distribution of anionic sites were similar to those observed in sham operated rats killed 6 weeks after surgery. These results indicate that glomerular sclerosis and GBM thickening are unrelated phenomena. They also suggest that the number and density of anionic sites in LD are not a prominent factor for increasing proteinuria after subtotal nephrectomy.