Using a method described in a previous article the transepidermal water loss (TEWL) was studied in 10 healthy newborn infants at rest and during activity. On the average TEWL was 37% higher during activity than during rest although no sweating was observed. In 9 infants placed in incubators with an ambient temperature slightly above the thermoneutral range measurements were made as the body temperature rose. TEWL was almost constant until a temperature of 37.1 degrees C was reached whereupon the water loss suddenly increased as the infant started sweating.