On the basis of our investigations in which for the first time the insensible water loss was measured directly in newborn infants during the first 24 hours of life, the magnitude of the heat loss from the total heat production due to evaporation of water was calculated. It was shown that this is lower than the 25% value obtained in infants and up to the adult age group. The amount of heat lost due to evaporation of water is dependant on the relative humidity of the surrounding air. In our investigations with the relative humidity in the incubator between 57 and 78%, 13–15% of the heat produced in the body was dissipated through the evaporation of water.