Effects of atrial fibrillation on coronary circulation and on blood flow distribution across the left ventricular wall were studied in anesthetized open-chest dogs. Atrial fibrillation was induced by pressing down mechanically the left atrial appendage or by stimulating electrically the left atrial appendage. Heated cross-thermocouples were used for measuring regional myocardial blood flow. The results showed a marked decrease in coronary blood flow with a significant increase in coronary vascular resistance; average values (SD) of flow and resistance during control simus rhythm were 71.6 +/- 7.36 ml/min . 100g heart muscle and 1.38 +/- 0.15 mmHg/ml/min . 100g heart muscle, respectively, and 54.0 +/- 13.60 ml/min . 100g and 1.54 +/- 0.18 mmHg/ml/min . 100gjust prior to cessation of atrial fibrillation. The termination of fibrillation caused a remarkable rise in coronary flow and a fall in coronary resistance. Pacing-induced tachycardia, similar to the average ventricular rate during atrial fibrillation, increased coronary blood flow and decreased coronary vascular resistance. These show that active coronary vasodilatation and an increase in extravascular support of coronary bed are produced by atrial fibrillation. Subendocardial myocardial blood flow during atrial fibrillation was reduced 22.0 +/- 14.8% from control levels followed by marked increase in flow after cessation of fibrillation, while subepicardial flow decreased only slightly. Thus, atrial fibrillation itself diminishes coronary flow reserve, especially in the subendocardial layer, partly due to the increase in myocardial component of coronary vascular resistance, and it is possible that irregular ventricular rhythm may play an important part in a rise in extravascular support.