In 68 infants appropriate for gestational age, born after 25-41 weeks of gestation, the evaporation rate from the skin and ambient, skin and body temperatures were measured repeatedly during the first 4 weeks after birth, and the heat exchange due to evaporation, radiation and convection was calculated. All measurements were made at an ambient humidity of 50%, with the infants calm and quiet. High evaporative heat losses were found in preterm infants on their first days of life, with gradually decreasing values with increasing gestational and postnatal age. It was only during the 1st week of life in infants born at 25-27 weeks of gestation that evaporative heat loss was higher than radiative heat loss. The high evaporative heat losses in very preterm infants were balanced by low losses or even a gain through radiation and convection, because of the high ambient temperature needed to maintain a stable body temperature. Total heat loss increased with increasing gestational and postnatal age.