The oxygen requirements of the chick embryo have been found to increase by at least 100% during hatching. This rise is probably a result of ventilating the newly functioning lungs and maintaining the body temperature at its pre-hatching level. Sustained homeothermic responses become evident only at the moment of escape the shell membranes. There are two essential phenomena in the termination of the embryonic existence. The first is the initiation of pulmonary respiration which appears to result from a high partial pressure of carbon dioxide in the blood stimulating the respiratory centres. Active hatching is the second essential phenomenon and is probably stimulated by an increase in the rate of thyroid hormone secretion. The body temperature of the chick rises after hatching, and is probably a result of the progressive replacement of the yolk with actively metabolizing tissues. There is also a rise in the metabolic rate at this time. Both the absolute oxygen requirements and the metabolic pattern of the growing fowl may be affected by the diet. There is a difference in the response to the diet according to the sex. A period of almost constant oxygen uptake was consistently noted and appeared to be virtually independent of the diet although high levels of dietary protein tended to reduce this depression in the metabolism. At 4 weeks of age a large fall in the metabolic rate was found, especially in RIR x IS chickens. The significance of the fall is not known, but a fundamental change in the physiology of the fowl is indicated.