The fine structure of the glomerular basement membrane (GBM) of the rat kidney was studied by means of high resolution scanning electron microscopy. Specimens were taken from kidneys perfused with paraformaldehyde, freeze-fractured and then processed with conductive staining. The fractured surface of glomerular tufts exhibited the inner and outer surface of the GBM uncovered by endothelial and epithelial cells. The lamina densa was composed of densely packed granular material together with scattered fibrils. The laminae rarae interna and externa were composed of a meshwork that showed some structural heterogeneities. The meshwork composing the lamina rara interna contained 5- to 9-nm-thick fibrils, had pores 11-30 nm wide, and was associated with granular material except in those places that corresponded with endothelial fenestrae. The meshwork of the lamina rara externa was made up of 6- to 11-nm-thick fibrils, and had smaller pores under the foot processes (10-24 nm wide) than those near the filtration slits (16-32 nm wide). In addition to the meshwork, the lamina rara interna contained microfibrils that were arranged differently depending on the topography of the capillary wall: scattered fibrils had no predominant orientation at the convex side, circumferential bundles lay at the concave side of the peripheral capillary wall, and had a circumferential arrangement in the paramesangial wall.