The excretion of methoxyflurane in end-expired air has been studied in patients after methoxyflurane anesthesia and in anesthesiologists after they have administered methoxyflurane anesthsia. Methoxyflurane is detectable in the end-expired air of patients for 10–18 days after anesthesia and in anesthesiologists for as long as 30 hours after exposure. Concentrations in patients' end-expired air could be related to a time-and-concentration measure, methoxyflurane per cent hours (MFPH), for as long as 120 hours after exposure. Samples of air collected in the area in the operating room from which the anesthesiologist inspired air contained 1.3–9.8 ppm methoxyflurane, depending on the concentration of anesthetic being delivered to the patient. A significant decrease in the methoxyflurane concentration in operating room air was obtained using a gas trap. Due to the potential hazards of chronic exposure to anesthetic gases, protection of the anesthesiologist and other operating room personnel seems advisable.