Partitional calorimetry was used to assess the influence of a double-walled incubator (Air-Shields C-86 Isolette) on the body heat loss of eight premature newborn infants (birth weights 1.44-1.89 kg, ages 6-19 days). Each infant was studied in the same incubator with and without the inner wall. Incubator heater output was regulated by servocontrol to maintain the abdominal skin temperature at 36.5 degrees C. Operative environmental temperature was the same (mean 33.0 degrees C) in both incubators. There were no differences in body temperatures, oxygen consumption, carbon dioxide production, respiratory quotient, or evaporative water and heat losses. The double-walled incubator reduced radiant heat loss but increased convective heat loss, so that the total rate of body heat loss was unchanged.