This study determined the relative contributions of systemic (bronchial) and pulmonary blood flow to the intraparenchymal airways greater than 1 mm in diameter using fifteen-micrometer fluorescent microspheres and fluorescence microscopy in four dogs. Fluorescent microspheres of one color were injected into the inferior vena cava as a pulmonary blood flow marker, and fluorescent microspheres of another color were injected into the left ventricle as a systemic blood flow marker. After the second injection the animals were killed, the lungs excised and air dried at total lung capacity. The left lung was sliced into transverse planes and then sectioned into smaller blocks containing airways down to 1 mm in diameter. The blocks were then sectioned using a Vibratome¨ and examined with a fluorescence microscope. Pulmonary and systemic blood flow markers were counted in airway walls and the diameter of each airway was measured to determine the bronchial tissue volume. After correcting for the number of blood flow markers injected into each circulation, the average ratio of pulmonary to systemic blood flow markers seen in airway walls was 1:37 indicating that 97% of the blood supply to the intraparenchymal airways down to 1 mm in diameter is from the bronchial circulation. Furthermore, based on a weighted least squares regression analysis, systemic (bronchial) blood flow per unit tissue volume increased as airway diameter decreased (p=0.03).