1. Oxygen consumption (V(O2)) has been measured serially in sixty-eight infants during the first 10-35 days of life, when naked in a Perspex metabolic chamber, by recording the changes in circulating gas volume. Air speed in the chamber was 4-5 cm/sec, and absolute humidity approximately 18 mm Hg.2. When environmental temperature (T(E)) was 35-38 degrees C minimal V(O2) rose from 5 to 7 ml.O(2)/kg.min during the first 2 days of life in infants weighing over 2.5 kg, and more slowly in the first 7-10 days in infants under 2 kg at birth.3. Physical activity and V(O2) both increased when T(E) fell below 33 degrees C: the increase appeared to be linearly and inversely related to T(E), but the rise in heat production was seldom enough to prevent a fall in rectal temperature. In infants over 2.5 kg at birth the mean increase amounted to 0.56 ml.O(2)/kg.min for each 1 degrees C fall in T(E) when 4-12 hours old, and 1.27 ml.O(2)/kg.min when between 4 and 20 days old. In infants weighing 1-2 kg at birth the mean increase was similar in the first 12 hr, but the coefficient rose more gradually with age.4. The maximum V(O2) in infants over 2.5 kg at birth and over 2 days old was about 2(1/2) times the minimum V(O2); the maximum was rather lower in most infants of low birth weight.5. In seven infants who were motionless and apparently asleep after sedation with chloral hydrate, the increase in V(O2) at low T(E) was reduced but still significant.6. It is concluded that the new-born baby responds to a cool environment with a considerable immediate increase in heat production; visible muscular activity appears to account for only part of this increase.