1. The relation between tissue respiration and body size was investigated in the rat. Determinations of Qo2 were made on heart, lung, liver, kidney cortex, brain cortex, diaphragm, and thymus of animals ranging from 9 gm. to 392 gm. body weight, including some determinations on fetuses and fetal tissues. A statistical evaluation of ca. 230 experiments is given.2. The diaphragm is the only organ investigated to show a definite and significant correlation between rate of tissue respiration and body size. Liver and thymus show a break in the regression line which corresponds to a number of other characteristic changes in metabolism and growth.3. The experiments do not show systemic differences in tissue respiration accounting for the decrease of total metabolic rate with increasing body size.4. A comparison between intraspecific and interspecific size-dependence of tissue metabolism is made.5. The current theories on the systematic decrease of weight-specific metabolic rate, as expressed in the surface or ¾ ...