Respiratory heat loss (RHL) were measured under thermal neutral helium-oxygen hyperbaric conditions until 101 bar on 2 cats and RHL was measured under atmospheric air andhyperbaric helium-oxygen conditions of 91 bar with variation of the inspired gas temperature(ambient temperature) on 3 cats.In spite of under thermal neutral zone, respiratory heat loss increased at high ambientpressure causing mainly by the augmentation of respiratory convective heat loss.The ratio of respiratory heat loss to metabolic heat production elevated from 10% at atmospheric air to 30∼35% at hyperbaric helium-oxygen (30∼35 bar), and under ambient pressure abcve 30∼40 barthi sratio is nearly constant 30∼35%.Helium-oxygen hyperbaric condition of 91 bar, the respiratory heat loss remarkablyincreased at cold environment and their ratio to metabolic heat production was 70∼80% at30°C inspired gas temperature (ambient temperature).If the relation between this ratio andinspired gas temperature were extrapolated to temperature of 27∼28°C, all heat producted bymetabolism is lossed by only respiration, in spite of increase of metabolic heat production atcold environment.Under low ambient temperature of helium-oxygen hyperbaric condition, the penetration ofcold inspired gas in the body core probably give the important influences on physiologicalfunctions.