Minute volume, tidal volume, respiratory frequency, end-tidal carbon dioxide, and ventilatory response to CO2 inhalation were studied during anesthesia at 3 different concentrations of end-tidal methoxyflurane, 0.19, 0.37, and 0.82 per cent, in 12 unpremedicated patients prior to surgery. Minute volume rose during light anesthesia and fell with increasing concentrations. Tidal volume decreased progressively from 0.49 liters awake, to 0.22 liters at the highest concentration, while respiratory rate rose from 11 to 22 per minute. End-tidal CO2 rose progressively from 38.6 torr while awake, to 43.4 torr during deep anesthesia. More striking was the change in slope of the CO2 response curve, which decreased progressively with increasing depth of anesthesia from 56% of the control response in light anesthesia, through 28 to 9% during deep anesthesia. An estimate of MAC [minimum anesthetic concentration] for methoxyflurane in this study was 0.17%, which agrees with previous reports despite employment of different criteria.