1. The purpose of the study was to see whether the rate of rise of alveolar PCO2 (PA, CO2) in expiration was directly proportional to the rate of pulmonary elimination of CO2 (VCO2) in man in the steady state. 2. Alveolar ventilation at rest and during exercise in man was calculated from the difference between total ventilation and dead space ventilation, and from the ratio of the rate of pulmonary CO2 elimination to the mean expired alveolar CO2 (total) fraction. The results were indistinguishable. In agreement with other workers' findings alveolar ventilation changed in direct proportion to the rate of carbon dioxide elimination, confirming the isocapnia of exercise ventilation in man. 3. The rate of rise of expiratory alveolar PCO2 in individual breaths has been obtained by two methods. In the first, a pattern of respiration with constant expiratory flow in each breath brought expiratory alveolar profiles to the outermost end of the airway. In the second method, the early part of the alveolar PCO2 during normal expiration was calculated from airway PCO2 and expired volume. 4. The data obtained with both methods show that, in the steady state, expiratory alveolar PCO2 rises at a rate which is directly proportional to the rate of CO2 production.