BASSETT, D. R. Jr. and T. R. FITTON. Gas mixing apparatus for determining cardiac output by CO2rebreathing.Med. Sci. Sports Exert:, Vol. 27, No. 12, pp. 1698–1702, 1995. The carbon dioxide rebreathing technique is widely used for determination of cardiac output during exercise. The equilibration method of Collier et al. (J. Appl. Physiol.9:25, 1956) is generally preferred over the exponential method of Defares (J. Appl. Physiol.13:159, 1968). However, the equilibration method requires the volume and initial CO2percentage in the rebreathing bag to be adjusted according to the work rate. A device for mixing two gases (100% O2and 20% CO2/80% O2) was constructed for this purpose. Multistage regulators are attached to the gas tanks and connected to a medical gas mixer via high-pressure air hoses. A variable time-delay switch causes a solenoid valve to open for 1.0–10 s to deliver a predetermined gas volume. The device was found to accurately deliver a preset volume and concentration of gas to the rebreathing bag. A gas mixing apparatus simplifies the equilibration CO2rebreathing technique by allowing the investigator to easily select the initial volume and percentage of CO2.