The relative importance of pCO2 versus pH in regulating myocardial blood-flow (MBF) is not settled. Therefore, the influence of hypocapnia, hypercapnia and sodium carbonate infusion, on MBF and myocardial metabolism, has been investigated in 10 closed-chest pentobarbital anaesthetized dogs. The animals were hyperventilated, and CO2 was added to the inspiratory gas to induce normocapnia and hypercapnia. A mass spectrograph continuously measured the ventilatory gas components, and MBF was measured by the hydrogen desaturation technique with a catheter positioned in the coronary sinus. During the experiments, there were no significant alterations in heart rate, mean aortic blood-pressure, myocardial oxygen consumption or uptake of glucose and free fatty acids. During hypocapnia MBF was insignificantly reduced, while myocardial oxygen extraction increased significantly. During hypercapnia, however, MBF increased more than 40%. This increase in MBF was abolished following an infusion of sodium carbonate. Thus, in the present study, increased MBF, observed during hypercapnia, was due to the reduction in pH and not to the increase in pCO2.