1. The rates of oxygen consumption were measured in 6-8-day-old rabbits at 34 and 15 degrees C after varying periods of starvation and cold exposure. At the start of the experiment the rabbits had been fasted for 24 hr. Eight rabbits were studied immediately, six after 24 and six after 48 hr in a cold environment (20 degrees C), and twelve after a further 48 hr in a warm environment (34 degrees C). All the animals had a similar increase in oxygen consumption during the final hour of cold exposure (15 degrees C).2. The rabbits kept at 20 degrees C lost 83% of the fat stored in their brown adipose tissue within 24 hr and a further 11% in the next 24 hr. The fat content of white adipose tissue had fallen by 75% at 48 hr. In contrast rabbits kept unfed at 34 degrees C had lost 47% of the fat in brown adipose tissue and 44% of the fat in white adipose tissue after 48 hr.3. In six rabbits subcutaneous thermocouples demonstrated that local heat production continued in brown adipose tissue after 48 hr cold exposure.4. In the rabbits kept at 34 degrees C the final cold exposure caused a large increase in the serum free fatty acid and glycerol concentrations. Much lower concentrations were found in rabbits kept at 20 degrees C.5. The results show that the fat stored in the brown adipose tissue of young rabbits exposed to cold is preferentially used for heat production. When this store of fat is exhausted, brown adipose tissue still produces heat presumably by oxidizing fat and glucose taken from the circulation.