A comparative evaluation of various types of humidifiers and heat and moisture exchangers was made on normal volunteer subjects and tracheostomized patients. The respiratory water loss during oral breathing in normal adults was 27.7 mg/liter of expired air which represents 75% saturation with water vapor at 32 C. It was directly proportional to the minute volumes. The net water loss from the respiratory tract decreased as the absolute humidity of the inspired air increased. Atropinized and anesthetized subjects expired 28.6 mg and 23.4 mg/liter of air. Rebreathing through an external dead space of 100 ml reduced normal respiratory water loss by 3 mg. A heat and moisture exchanger reduced the normal respiratory water loss by one half. Under the usual conditions employed, with oxygen tents or nebulizers, subjects were in negative water balance.