Excretion of methoxyflurane was studied in 12 patients receiving anesthesia in a closed rebreathing circuit at a constant alveolar concentration of approximately 0.24 per cent. The mean methoxyflurane uptake was 18 g (range 7.6-31 g) during a mean time of anesthesia administration of 2 hours, 18 minutes (range 55-309 minutes). An average of 19 per cent of the uptake was recovered unchanged in the exhaled air after anesthesia. Urinary excretion of organic fluorine, fluoride, and oxalic acid was equivalent to 29, 7.7 and 7.1 per cent of methoxyflurane uptake, respectively. Approximately a third of the uptake remained unrecovered. It is postulated that a portion of the unrecovered drug became permanently bound to tissues and hence its excretion was delayed beyond the period of the study.