The effect of hypocapnia on global and regional myocardial function and coronary blood flow (CBF) was studied in dogs anesthetized with halothane before and after critical constriction of the left anterior descending (LAD) coronary artery. Coronary blood flow decreased 29% (P less than 0.05) when hypocapnia was induced in dogs with a normal LAD artery. Critical constriction reduced CBF by 42% (P less than 0.05). In the critically constricted LAD artery, hypocapnia did not reduce CBF. During hypocapnia and with a normal LAD artery, oxygen extraction by the myocardium increased 16% (P less than 0.01) and oxygen tension in the coronary sinus was reduced by 19% (P less than 0.001). After critical constriction of the LAD, hypocapnia was associated with an increase in oxygen extraction of 14% (P less than 0.01) and the coronary sinus oxygen tension was reduced by 21% (P less than 0.001). CBF of the left circumflex coronary artery (LC) increased 36% (P less than 0.05) after critical constriction to the LAD when compared with control values of the preconstriction phase. However, LC flow did not change during hypocapnia when critical stenosis had been applied to the LAD artery. Although oxygen supply (product of CBF and arterial oxygen content) to the myocardium was reduced during hypocapnia, regional myocardial function did not change from control values. Regional function was similarly maintained during hypocapnia and critical constriction of the LAD.