We are reporting an electronic circuit which uses the peak end-tidal CO2 signal from a rapid infrared CO2 analyzer to vary the motor rate of a fixed volume respirator. It contains variable gain and a lag compensation network which permits critical damping to prevent oscillation. The CO2 analyzer, circuitry, and respirator are connected in a closed-loop servo system that allows automatic control of the CO2 level. The system's gain and performance are such that it can accommodate large changes of CO2 return to the lungs with no more than +/- 0.5 Torr carbon dioxide pressure (PCO2) error signal. It has proved useful in experiments on neural respiratory control in paralyzed animals where it is desired to keep PCO2 constant despite changes in cardiac output and venous and CO2 return to the lungs, and to monitor the approximate magnitude of these changes.