There have been many reports on changes in renal morphology with aging. In this study, morphological comparisons were made on the influence of aging in humans and rats. Kidneys were obtained from 63 autopsies (except those from cases of tumor, severe cicatrization, and cysts 10 mm or more in diameter) performed on humans aged from 0 to 92 years, including 7 cases with a past history of hypertension; the findings were compared with those from 201 Wistar male rats aged three to 115 weeks. First, changes in renal weight and renal cortex width were investigated. Next, changes in renal morphology were examined by microscopy. Finally, after performing alcian-blue-PAS staining, the ratio of the intima/outer diameter (I/OD ratio) of renal blood vessel was measured under a microscope with an image analyzer, and the results compared. A decrease in renal weight was observed in humans from about 50 years of age, but there was no decrease in weight with aging among rats. While significant thickening in the vascular intima was observed with aging in humans, there was no such thickening among rats. Furthermore, the presence of hypertension caused renal cortex width to be significantly decreased in humans (p < 0.04). Findings of sclerogenous changes of the glomeruli, infiltration of chronic inflammatory cells and fibrosis in the stroma, and tubular casts were observed with aging in both humans and rats. In conclusion, arteriosclerosis apparently develops with aging in humans, suggesting that arteriosclerosis greatly influences the reduction of human renal weight.