Theoretically, the airway circulation can regulate the absorption of inhaled drugs, redistribute inhaled drugs within the respiratory tract, and influence the distribution of systemic drugs to the airway. Some of these functions of the airway circulation have been verified experimentally, but not in human subjects. Since pharmacological manipulation of airway blood flow could have therapeutic implications in airway disease, the development of new methods to measure airway blood flow noninvasively in human subjects is needed.