The influence of environmental humidity and temperature on the thermal balance of 45 full-term newborn infants was studied by direct calorimetry within 24 hours after birth. The respiratory heat loss measured at 32C and 20% relative humidity (RH) represented 9.5% of the total heat production, and it decreased to 2.9% when RH was 80%. In neutral thermal environment (32C, 50% RH), the mean respiratory heat loss was lower than that measured during a warm exposure (36C, 50% RH), in spite of a higher absolute humidity in the latter condition. This suggests that respiration might have a thermoregulatory function during heat exposure in the newborn. Evaporative heat loss from the skin was more elevated than that from the respiratory tract, but it was less sensitive to change in ambient humidity. Convective and radiative heat losses from the skin were inversely related to ambient temperature; similarly, the metabolic rate decreased with increasing ambient temperature up to 36C. This work provides further data on the varying energy exchange between the newborn infant and his environment; it should lead to a more rational planning of infant care and caloric requirements and demonstrates the important effects of different environmental conditions on the newborn infant.