1. The ratios between the rates of growth of the body and of the heart, kidneys, and liver are approximately uniform between 40 gm. body weight and the body weight at maturity in the albino rat. The male and female hearts grow at 0.75 times the rate of growth of the body, the male kidneys at 0.717 times, the female kidneys at 0.648 times, and the liver at 0.838 times the rate of growth of the body as a whole.

2. Formulas for the prediction of organ weight from body weight were derived from the data on 1591 albino rats kept under constant conditions.

3. A series of experiments in which dietetic and metabolic variables were introduced into otherwise constant conditions showed that the heart weight was not affected by diet, and that both kidney weight and weight of liver protein (used as a measure of effective liver size) varied in the direction of change in the protein content of the diet. Decrease in rate of metabolism induced by thyroidectomy and increase in metabolism following the administration of thyroxin led to a corresponding fall and rise of heart, kidney, and liver protein weight. These results were confirmed in experiments on fasted rats with the exception that under these conditions thyroidectomy did not appreciably decrease liver protein weight relatively to fasted controls. Increase in organ metabolism due to dinitrophenol had no effect on organ weight.

4. When experimental changes alter the composition of the body with respect to fat or water, the comparison of experimental and control organ weights in terms of any one function of body weight is fallacious.

5. Conditions that change kidney weight usually change liver protein weight in the same direction and roughly to the same degree. The possible meaning of two exceptions to this rule is discussed.

6. The observations made are regarded as supporting the hypothesis that, after weaning, change in the weight of the heart, kidney, and liver protein is determined mainly by change in the amount of work done by these organs.

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