The heat production of the albino rat, measured 17 hours after food, decreases as the temperature becomes warmer and at 28° C. and above is essentially constant, thus indicating the critical temperature at which measurements of the basal metabolism should be made. Season has a distinct effect upon the rat's metabolism, for the heat production is perceptibly lower (on the average 10 to 12 per cent) in the sumer than in the winter. This effect is explained perhaps wholly by the difference in envitonmental temperature. Rats studied at 28° C. after living 24 hours at this same temperature have a lower heat production (on the average 4.5 per cent) than rats studied at 28° C. after living 24 hours at 21° C. Female rats over 2 months old, mated but not studied during pregnancy or lactation, showed an increasing heat production with advancing age, both at 26° and at 29° C. With male rats the data, although not so numerous, especially beyond the age of 12 months, indicate the same trend in metabolism. The increasing metabolism with these rats cannot be ascribed to increased activity, to lactation, or to pregnancy. The repeated periods of pregnancy and lactation during the life of the normal female rat would, however, be expected to result in a lower metabolism during old age. Experiments to throw light on this problem are needed with both mated and unmated females during advancing age. Male rats have a distinctly higher metabolism than femal rats, up to at least 14 months of age, both at 25° and at 29° C. A tremendous decrease in metabolism was noted with three rats which were studied up to within a few hours of death. THe strikingly high or how metabolism observed consistently in a few individual cases stresses the importance of careful autopsies and indicates the value of having some sort of standard for the rat's basal metabolism, in order to recognize quickly the possibility of an animal being physiologically abnormal. The female albino rat, 2 months old and over, at 28° C., has a basal metabolism, with cage activity, averaging about 800 calories per squeare meter of body surface per 24 hours, and with complete muscular repose averaging approximately 720 calories. The basal heat production of male rats of the same ages may be estimated to average about 800 calories, with complete repose. Thus, in spite of its shorter life cycle, small size, and high heart rate, the rat has a metabolism on the body surface basis distinctly low as compared with that of humans. In comparative physiology the influence upon metabolism of both the active mass of protoplasmic tissue and the stimulus to this mass should be taken into consideration. The heart rate and the blood volume may possibly serve as approximate measures of this stimulus.