Changes of pulse rate and arterial blood gases were measured before, during, and after fiberoptic bronchoscopy in eleven patients with resting arterial PO2 breathing room air of 75 mm Hg or greater. Tachycardia was greatest after anesthesia (mean change greater than 20 beats/min) and declined thereafter. Hypoxemia was progressive from the time of entrance of the bronchoscope into the respiratory tree and continued into the immediate postbronchoscopic period when the mean fall was about 16 mm Hg. Mild hyperventilation was noted under all conditions. Since progressive hypoxemia persists following removal of the instrument and does not occur during rigid bronchoscopy, direct irritation causing reflex bronchospasm and the effects of aspirated secretions, anesthetic or lavage must be considered as possible causes for the hypoxemia.