A comparison of ventilatory tests and the response to carbon dioxide rebreathing was made in healthy smokers, nonsmokers, and in subjects with bronchitis. The response to carbon dioxide (CO2) rebreathing was the same in the healthy population but diminished in the bronchitic group. Effects of smoking on maximal expiratory flow rates did not correlate with the results of carbon dioxide rebreathing. A consistent pattern of relationship of ventilation response to carbon dioxide and age of the subject was apparent.