Simultaneous measurements of the rates of heat production and oxygen utilization by living tissues provides a direct quantitative means of detecting subtle changes in metabolic state in the face of altered physiological and environmental conditions. The ratio of calorimetrically measured heat flux to the respirometrically measured oxygen flux is often called the “calorimetric-respirometric ratio” or CR ratio. This can be used to partition total metabolic energy flux into its aerobic and anaerobic components by comparison of the experimental CR ratio with the theoretical “oxycaloric equivalent” for fully aerobic respiration. Such information is most important when evaluating the physiological effect of environmental stress.