The relative amount of interstitial cortical tissue was measured by the point count method in kidney tissue from human individuals without renal disease. One series (54 kidneys) consisted of kidneys intended for transplantation and removed immediately after death from persons who died suddenly. The other series (69 kidneys) was obtained by autopsy. In both groups, the percentage of interstitial tissue was dependent on age and followed the equations of regression (1) y=12.45+0.11 x (donor series) and (2) y=23.8+0,10 x (autopsy series). The autopsy values were significantly greater than the donor-kidney-values. There was no difference due to sex. The relative number of sclerotic, obsolescent glomeruli was very small (0–1%) until the age of 40. Thereafter it increased, most markedly in the autopsy series, until it reached values of about 30% in persons more than 80 years old.