1. The effect of breathing a mixture of 5–6% carbon dioxide in air was examined in seven patients with obstructive bronchitis and in eight patients with lung disease but without bronchial obstruction.
2. Hyperventilation during CO2 breathing was less marked in patients with bronchial obstruction than in the others. Cardiac output did not change in patients with airways obstruction but rose in the control group. The increase of arterial carbon dioxide tension, hydrogen ion concentration, pulmonary arterial and pulmonary wedge pressures (P̄pa and P̄paw was comparable in both groups. The increase of P̄pa and P̄paw was proportional to the Pa,co2 but the change of cardiac output was not. The ventilatory response to CO2 breathing was correlated with the spirometric measurements, but the haemodynamic changes were not.
3. No significant changes of pulmonary vascular resistance or of the P̄pa–P̄paw pressure gradient were observed. The increase of pulmonary arterial pressure during CO2 breathing appears to be due to increased pulmonary wedge pressure.
4. The contribution of depressed myocardial function during CO2 breathing to the response of cardiac output is discussed on the basis of a negative correlation between Pa,co2 and stroke volume during CO2 breathing.