1. Rabbits were reared from birth under one of two contrasting conditions. They were either well-fed and kept in a cool environment, or kept warm but fed on alternate days only. When they were a week old their metabolic responses to cold or the net exchanges of glucose, free fatty acid and glycerol by their brown adipose tissue in vivo before and during noradrenaline infusions were measured.2. The cold-reared rabbits grew normally, the under-fed rabbits were undersized; in both groups the brown adipose tissue wet weights were less than those normally found at birth. In cold-reared rabbits this was due largely to the low fat content, in the under-fed rabbits it was not.3. The rabbits' rates of oxygen consumption at a thermoneutral temperature and in response to cold was greater in the cold-reared rabbits.4. The increase in temperature over brown adipose tissue and the increase in blood flow in response to noradrenaline infusion was also greater in the cold-reared rabbits.5. Although the brown adipose tissue of cold-reared rabbits contained 30% fat it did not release fatty acids 24 hr after a feed. In contrast the brown adipose tissue of the under-fed rabbits released fatty acids at a high rate.6. It was concluded that both prolonged cold exposure and undernutrition influenced the function and development of brown adipose tissue during the first week of life. Cold exposure increases the thermogenic capacity of the tissue, undernutrition reduces it. The net release of fatty acids from brown adipose tissue, is, in part, dependent on the tissues' own intracellular requirements.
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