The changes produced by atrial fibrillation on myocardial blood flow and energetics are unclear. Accordingly, the effects of atrial fibrillation on myocardial blood flow and oxygen consumption were studied in 18 anesthetized dogs. With atrial fibrillation cardiac output and mean aortic pressure declined. Although average myocardial blood flow and oxygen consumption did not change, the alterations in these variables correlated strongly (r = 0.95, P less than 0.01). Moreover, myocardial blood flow changes correlated with those of tension time index and peak dp/dt (r = 0.64, P less than 0.05). Distribution of left ventricular regional flow remained uniform irrespective of the directional changes in blood flow. However, average left ventricular inner to outer wall flow ratio declined and myocardial oxygen extraction increased. Thus, with atrial fibrillation, myocardial blood flow does not change in a consistent fashion, although myocardial blood flow still reflects myocardial oxygen demand. Relative hypoperfusion of subendocardium and an increase in myocardial oxygen extraction suggest that with atrial fibrillation the myocardial blood flow response is abnormal.