In the study of the physiological regulation of respiration through a control system model it is necessary to test the ventilatory response to various forcing functions of either the parrtial pressure of alveolar carbon dioxide (PACO2) or oxygen (PAO2). Since PACO2 and PAO2 are both functions of alveolar ventilation and metabolic rate, such a result cannot be obtained by merely changing the composition of the inspired gases without a feedback control. Thus a servomechanism is necessary. The input to the servomechanism is an instantaneous determination of PACO2 and PAO2. This is accomplished by using the criterion of equality of the exchange ratio in mean alveolar gas and mean expired gas. The servomechanism described has three essential characteristics: rapidity, accuracy, and stability. In experiments of step, ramp, and sinusoidal forcing functions, variations of PACO2 have been obtained without change in PAO2, and step variations of PAO2 have been obtained without change in PACO2.