Comparative study has been made histologically and micrometrically on the senile changes of the kidney between Caucasians and Japanese. In both races, weight loss of the kidney, decrease in number of the parenchymal cells and tendency to increase in size of the cells were considered to be main characteristics of the senile changes, but in the process of the senile changes some differences were noticed between the two races. The race differences may be partly due to the difference in the process of sclerotic changes of the intrarenal arteries. These differences have been discussed with special consideration to the nutritional conditions of the two races.