The temperatures (TI, TE) of inspired and expired gas and the mass of expired water (MEH2O) have been measured in four subjects at rest during mouth and nose breathing of dry air at room temperature. TI and TE were measured by copper-constantan thermocouples, MEH2O by freezing and ventilatory parameters by total body plethysmography. During mouth breathing, temperatures are significantly higher (TI = 28.1 degrees C, TE = 31.5 degrees C) and the amount of expired water larger (MEH2O = 27.8 mg dm-3 BTPS) than during nose breathing (TI = 24.8 degrees C; TE = 29.6 degrees C; MEH2O = 26.6 mg dm-3 BTPS). From these experimental data the appropriate computations show clearly that in humans, while either nose or mouth breathing, the expired air is not water saturated; the latent heat exchanges represent the larger part of the respiratory heat exchanges; the counter current expiratory heat recovery is imperfect; in terms of heat and water respiratory exchanges, no large difference exists between the oral and nasal routes. This last point is confirmed by the calculation of a difference less than 10% in the total respiratory heat losses between mouth and nose breathing.

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