The need of continuous methods of recording changes in expired CO2 and O2 for studying disturbances in equilibrium in the absorption, elimination, and transport of these gases, as contrasted with a similar study under equilibrium conditions, is discussed. The method devised for recording changes in CO2 tension consisted of a fine MnO2 electrode resting on a peritoneal membrane moistened with KC1 in electrolytic connection with a calomel electrode. This unit, called the CO2 electrode, was exposed to a steady flow of expired air. Increased or decreased tension of CO2 led to increased or decreased acidity of the CO2 electrode. Resulting changes in E. M. F. were balanced with a type K Leeds and Northrup potentiometer and mechanically recorded on smoked paper. The method devised for recording changes in O2 tension of the expired air consisted of a Bunsen burner supplied with a constant flow of combustible gas and expired air. Changes in O2 tension of the expired air led to changes in intensity of the heat of the flame, the effects of which on a constant flow of water were recorded with a sensitive thermopile. The changing E. M. F. was balanced and recorded as in the CO2 method. In vitro experiments demonstrated the accuracy and reliability of these methods. A rapid means of calibrating during animal experiments was described and the advantages of the methods were demonstrated by records from experiments.