The systemic arterial blood supply to the lung (bronchial blood flow, Qbr) is commonly measured using the reference flow technique by injecting radiolabeled microspheres into the left atrium (LA) and simultaneously collecting a reference blood sample from a large artery such as the aorta. These measurements are often done with the pulmonary artery occluded prior to the time of injection. We have reported previously that left atrial blood refluxes up the pulmonary veins following pulmonary artery (PA) occlusion. We designed this experiment to determine if 15 mu microspheres reflux from the LA into the left lung (i) when the left PA is occluded prior to microsphere injection and (ii) when the left PA is open during the injection. We calculated Qbr to the left lung after simultaneous left ventricular (LV) and LA injections of different radiolabeled microspheres in anesthetized, closed-chest, prone goats. When the PA was open (but occluded 5 sec after the microsphere injection), Qbr calculated from an LA injection differed little from that calculated from an LV injection of microspheres. However, when the PA was occluded prior to injection of the microspheres, Qbr calculated from an LA injection was significantly higher than that calculated from an LV injection suggesting that microspheres reflux from the LA to the lung.