The systemic circulation to the lung supplies the trachea and airway walls and may be important in the pathophysiology of asthma and pulmonary oedema. An understanding of the venous drainage pathways of this bronchial blood flow may be therapeutically important. The purpose of this study was to understand the normal drainage pathways in sheep. In seven anaesthetized, ventilated sheep we injected echo contrast agents into a systemic vein or into the bronchial artery while performing echocardiography to determine whether the drainage could be observed to the right heart and/or to the left heart. During transoesophageal echo (n=5) or heart surface echo (n=2), cephalic vein injection of <8 microm diameter gelatin microballoons promptly opacified the right but never the left-sided circulation. Air in agitated saline in the seven animals showed the same result. By contrast, injection into the bronchial artery promptly opacified the left atrium, left ventricle, and aorta but not the right-sided circulation in all seven microballoon injections and all but one of the air in agitated saline injections. The failure of the echo agents to pass through the pulmonary circulation may be related to sheep pulmonary intravascular macrophages or the surface forces on air bubbles of small size promoting collapse. The main conclusion is that there are bronchopulmonary anastomoses that connect the bronchial circulation to the pulmonary venous circulation connecting distal to the pulmonary capillaries. Any bronchial venous drainage to the right-sided circulation must have been below the detection level of the instruments and would in any case appear to be much less that the post-pulmonary capillary anastomoses noted. Pulmonary venous hypertension would be expected to have a direct effect on the bronchial circulation.