1. The left lower lobe of the lungs of six anaesthetized dogs were isolated by the introduction of a bronchial cannula at thoracotomy. Catheters were introduced into the main pulmonary artery and a vein draining the isolated lobe. 2. Blood-gas pressures and pH were measured across the isolated lobe and compared with gas pressures in alveolar samples from the lobe. 3. When the isolated lobe was allowed to reach gaseous equilibrium with pulmonary arterial blood for 30 min, there was no significant difference between alveolar and pulmonary venous PCO2. Mean values of whole-blood base excess were similar in pulmonary arterial and pulmonary venous blood. 4. After injection of 20 ml of 8.4% sodium bicarbonate solution into a peripheral vein, PCO2, pH and plasma bicarbonate concentrations rose in the mixed venous blood. There was no change of whole-blood base excess across the lung, indicating that HCO3-, as distinct from dissolved CO2, did not enter lung tissue in measurable amounts. 5. No systematic alveolar-pulmonary venous PCO2 differences were demonstrated in this preparation other than those explicable by maldistribution of lobar blood flow.