The structural changes of the renal glomeruli in development and aging were investigated by electron microscopy in normal rats from 4 to 36 weeks of age at 4 weeks intervals after birth. Infantile rats had an uneven glomerular basement membrane which was characterized by double-layered or mossy appearances of the lamina densa. On the other hand, diffuse and hemispherical thickenings of the basement membrane were observed in older rats. These thickenings were due to the accumulation of basement membrane materials on the epithelial side of the lamina densa. In the older rats, laminae rarae interna and externa were extremely thin. Epithelial cells of the young rat had abundant cytoplasmic organelles and relatively electron-lucent secretory bodies, whereas those of the older rat had less numerous cytoplasmic organelles, many myelinated bodies suggesting autophagosomes, and electron-dense whorl-formed secretory bodies. Mesangial regions of the young rat took small parts of the glomeruli and loosely contacted with endothelial cells. With increasing age, the mesangial matrix also took a greater part of the mesangial regions. The most even figure of the basement membrane was observed in 12 to 16 week old rats.