Little is known about normal variations and control of bronchial blood flow and bronchovascular resistance. We have used the reference-flow technique and 15-microns-diameter microspheres to measure bronchial blood flow under physiological conditions. Dogs (n = 13) were anesthetized and ventilated, and their chests were opened. A ligature was placed loosely around the left main pulmonary artery, and the left atrium was cannulated. In six dogs three sets of microspheres were injected simultaneously into the left atrium, and in another seven dogs the three sets of microspheres were injected sequentially at 0.5-h intervals. Prior to each injection measurements of pulmonary arterial, left atrial, and aortic pressures, cardiac output, and blood gases were made. Five seconds after injection the left main pulmonary artery was transiently occluded to prevent recirculation. After the final injection, dogs were killed, the lungs were removed, and the parenchyma was stripped off the large and small airways of the left lung. Knowing the radioactivity in the trachea, bronchi, parenchyma, and in the reference flow blood and also the aortic and left atrial pressures, we calculated bronchial blood flow (ml X min-1 X g dry lung-1) and bronchovascular resistance (cmH2O X ml-1 X min X 100 g dry lung). Results showed that there were no significant differences between the three measurements of bronchial blood flow when microspheres were injected simultaneously or sequentially. Bronchial blood flow to the left lung was 0.4% of cardiac output; 55% of the total flow went to lung parenchyma and 45% to trachea and bronchi. Expressed as flow/g dry lung the greatest flow was to the airways.