Seven groups of rats were fed on diets containing protein varying in amount from 1.36 to 40.13 per cent derived mainly from grain, casein, meat, and milk with carbohydrate, fat, and vitamines. From a number of animals one kidney was removed to double the load on the remaining one. The time of feeding was from 9 weeks to 6 months. Blood uric acid, blood urea nitrogen determinations, and microscopic examinations of the kidneys revealed no evidence of kidney damage. There was evidence of kidney hypertrophy consisting of increased weight of the kidney, large diameters of the capillary tufts, convoluted tubules, and kidneys in the animals receiving high protein diet. The nephrectomized animals that ate high protein had no kidney changes save hypertrophy and this amounted to an increase in weight of an average of 0.54 gm. or 85 per cent of the average weight of the right kidneys of the controls.