1. One hundred and seventeen healthy new-born babies weighing between 0·9 and 4·8 kg at delivery have been studied during the first ten days of life, and sixteen of these babies have been studied serially for 6 weeks after birth. The babies lay supine in a draught-free environment (air speed 4-5 cm/sec) of moderate humidity. The operative temperature was between 26 and 38° C for the babies who were studied naked.