Constant age-related increases in the width of glomerular basement membrane of normal rats were ascertained by quantitative estimation on electron micrographs. The thickness of glomerular basement membrane associated with aging was expressed by a regression equation; Y=0.03X+0.96 (Y, thickness of the glomerular basement membrane in 10(3) X A; X, rat's age in weeks), and the correlation coefficient between age and basement membrane thickness was very high (r=0.95). Standard deviation of the average thickness of glomerular basement membrane also increased with aging. Individual variation in various age groups was large in the infantile and older groups, while it was smallest in the group of 16 weeks of age. Frequency distribution curves of the thickness of basement membrane in the infantile and young rats were steep, whereas those in the older rats were blunt. The peak of curves of the older rats shifted to the right.