Abstract. Using a gradient layer direct calorimeter, total heat losses were measured in 69 full term new‐borns at 5 different ambient temperatures (TA): 28, 30, 32, 34 and 36° C. The relative humidity of the air was kept constant at 50%. Oesophageal temperature (Teg) and mean skin temperature (T̄s) were continuously recorded. All experiments lasted at least one hour. The mean total heat loss was at 28° C: 3.55 W/kg; at 30° C: 2.97 W/kg; at 32° C: 2.35 W/kg; at 34° C: 1.92 W/kg and at 36° C: 2.05 W/kg. Dry heat loss was proportional to the external temperature gradient. Evaporative heat loss was constant when new‐borns were not subjected to heat or cold stress, with a mean of 0.39 W/kg. This value is a measurement of insensible perspiration. Sweating was elicited at a TA of 36° C when the internal temperature gradient reached a mean value of 0.68° C. Heat storage (S). was calculated and was found to be negative at a TA of 28 and 30° C, and positive at 34 and 36° C. A regression analysis between heat storage and total heat loss showed a mean heat loss of 2.61 W/kg when the new‐borns were in thermal equilibrium (S= 0). This occured in most babies at a TA slightly below 32° C. A minimal metabolic rate of about 6.0 ml kg‐1 min‐1, determined by direct calorimetry and calculation of S, was found at a TA of 34 and 36° C. The cutaneous thermal conductance, which is an index of cutaneous blood flow was minimal at a TA of 30° C. It increased sharply when T̄s reached 36.3° C. These data indicate that the range of thermal comfort on the first day of life in normal full‐term new‐borns is very narrow and that there is a perfect thermal balance at a TA slightly below 32° C. However, at this TA, the metabolic rate is not at a minimal level.

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