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1. Measurements of total evaporative water loss (EWL) were made on sixty-three premature and full-term babies 0-65 days after birth within a closed Perspex chamber under varied environmental conditions by measuring the flow and absolute humidity of air entering and leaving the chamber. Control experiments suggested that the method underestimated loss by about 5%. Measurements of O₂ consumption were made concurrently by recording the volume change of the closed gas circuit.

2. The total basal EWL averaged 10·8 g H₂O/m².hr in infants 2-10 days old when ambient water vapour pressure (P_H2O) was 18 mm Hg; basal EWL was correlated with basal metabolic rate at all ages studied and evaporative heat loss accounted for 23% of basal heat production.

3. Respiratory water loss was measured by detecting the water added to air at 33° C passed across the face of eight infants in a trunk plethysmograph. Respiratory water loss was inversely related to the water vapour content of the inspired air; gas appeared to leave the nose 95% sat. at 35·6° C.

4. Measurements of total EWL were obtained when humidity was varied and skin loss was calculated by subtracting estimated respiratory loss from total loss; changing P_H2O from 7 to 25 mm Hg appeared to decrease basal skin water loss by only 1·5 g/m².hr.

5. No consistent changes in EWL were obtained when environmental temperature (T_E) was varied between 28 and 34° C. Active sweating occurred in infants 0-10 days old born within 3 weeks of term when T_E exceeded 34-35° C and rectal temperature (T_R) rose above 37·2° C. The threshold rectal temperature at which sweating was detected fell significantly in the first 10 days of life; EWL increased two to fourfold when T_R rose to between 37·5 and 37·8° C.

6. In infants of less than 215 days post-conceptual age (term 268 days) EWL increased less than 50% at T_R 37·7-37·8° C; it is concluded that the sweating mechanism is defective in these infants.