The ventilatory effects of Forane were studied in ten volunteers and compared with values obtained in eight volunteers anesthetized with halothane. Paco2averaged 60 torr with both 1.9 per cent alveolar Forane (approximately 1.45 X MAC) and 1.6 per cent halothane (1.9 X MAC). The slopes of the CO2response curves were depressed to 30 ± 6 per cent (Mean ± SE) of awake control values by 1.28 per cent Forane (approximately 1.0 X MAC) and to 45 ± 7 per cent of controls with 1.05 per cent halothane (1.25 X MAC). Therefore, when equivalent anesthetic doses are considered, less Forane than halothane was needed to increase Paco2and depress the slope of the CO2response curve. In contrast to halothane, Forane in increasing concentrations did not cause progressive increases in respiratory frequency. At equal multiples of MAC, Forane produces more profound respiratory depression than halothane, and this depression results from a unique failure of respiratory frequency to increase with increasing depth of anesthesia.