Oxygen consumption (Vo2), carbon dioxide production (Vco2), and insensible water loss (IWL) were measured simultaneously in nine nondistressed, appropriately grown, premature infants less than 2 weeks old, nursed in a conventional, blow-warmed incubator, and were compared with measurements made on the same infants under a radiant heater. The infants had a pronounced increase (148% on average) in IWL when under the radiant heater (P < .001) whereas Vo2 increased by only 4.6% (P = .073). Abdominal skin temperature (servocontrolled to maintain 36.5 C) and esophageal temperature were the same under both conditions, but ambient air temperature was 0.7 C higher in the incubator (P < .05). Although a positive correlation was found between the increase in IWL and the change in Vo2 (r = .75, P < .01), the large increase in IWL (and, therefore, evaporative heat loss) under the radiant heater is out of proportion to, and cannot be accounted for, by the change in metabolic heat production. The heat transfer processes involved in maintaining body temperature constant under these conditions require further study.