The marked fall in body temperature of premature infants immediately after birth and their notable tendency to be poikilothermic have led to the widespread practice of elevating and stabilizing the body temperatures of these infants by artificial regulation of ambient temperature and humidity. The selection of the ideal atmospheric conditions and of the most suitable body temperature has been made without knowledge of heat production by these infants during the critical first few days after birth. Most recommendations concerning the proper ambient temperature and humidity in the immediate postnatal period have rested on empirical grounds and on studies made on newborn animals and older premature infants, all of whom were presumed to be healthy.1-3 A good many premature infants suffer from