Abstract: Inhalation methods and apparatus are provided for performing xenon-enhanced studies or tests. The present invention is useful during computed tomography studies to provide neurologists with accurate yet relatively inexpensive measurements of cerebral blood flow. The apparatus includes a variable volume xenon/oxygen chamber for obtaining a closed-loop arrangement with the patient, thereby minimizing xenon gas usage during the study. According to the method of the invention, the volume of the xenon/oxygen chamber is reduced during a first phase of the study, while the patient's lung xenon concentration increases. The second phase is initiated when the variable chamber obtains a selected minimum value, after which exhalation returns to the chamber. During the second phase, oxygen and xenon are added to the closed loop system to maintain the desired concentrations, carbon dioxide is removed, and the chamber volume slowly increases.