This study investigated the effect of hypoxia on global and regional myocardial function in the normal heart and in the heart with a critically constricted coronary artery. Eight dogs under halothane anesthesia were subjected to progressive degrees of hypoxia while global and segmental heart function as well as general hemodynamics were evaluated. In the control phase of the experiment, the coronary circulation was undisturbed. A decrease in the inspired oxygen fraction from 0.3 to 0.1 was associated with significant increases in the heart rate, cardiac output, left ventricular minute work and coronary blood flow, and significant decreases in systemic vascular resistance. regional myocardial function (measured by endocardial fiber shortening) did not change. In dogs with a critical constriction applied to the left anterior descending coronary artery (LAD), hypoxia significantly increased heart rate and decreased stroke volume. Indices of global myocardial function did not change, but regional function in the area supplied by constricted LAD was depressed during severe hypoxia. This study implies that measured indices of global heart function bear little relation to regional function when a compromised segment of the myocardium is subjected to hypoxia.