The blood flow contributions and drainage patterns of the pulmonary and systemic circulations in the upper airways (trachea and main bronchi) were assessed in anesthetized dogs by injecting 15-micron radiolabeled microspheres into the right and left heart, respectively. After the animals were killed, the tracheal cartilage, tracheal muscle-mucosa, and main bronchi were excised. The tracheal cartilage and tracheal muscle-mucosa were divided into lower, middle, and upper segments for blood flow determinations. The pulmonary contribution to tracheal blood flow was very small, averaging only 0.6 +/- 0.3 ml.min-1.100 g-1 (means +/- SE), being higher in the lower segments. The systemic contribution to these same tracheal regions was significantly (P less than 0.05) higher, averaging 20.9 +/- 5.5 ml.min-1.100 g-1 (means +/- SE) and higher in the upper segments. The pulmonary and systemic circulations each contributed approximately 50% to the main bronchi blood flow, which averaged 11.2 +/- 4.2 ml.min-1.100 g-1 (means +/- SE). The pulmonary blood flow contribution alone to the trachea and main bronchi was also determined in subsequent experiments that utilized the isolated lung, and these blood flows were not significantly different from the pulmonary contribution measured in the intact lungs. The present results indicate that the systemic (bronchial) circulation is the primary source of tracheal blood flow and that both the pulmonary and systemic circulations may contribute approximately 50% of the blood flow to the main bronchi in dog lungs.