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1. Noradrenaline infusions (I.V. for 30 min) in new-born and 1-week-old unanaesthetized rabbits caused a rapid and sustained rise in oxygen consumption and an increase in blood glucose, free fatty acid and glycerol concentrations. Similar changes occurred in anaesthetized 1-week-old rabbits.

2. A simple direct method was used to measure the net exchange of metabolites across brown adipose tissue in anaesthetized young rabbits. It was found that the brown adipose tissue of rabbits reared from birth in a thermoneutral environment and studied when their next feed was due took glucose from and released fatty acids and glycerol into the circulation.

3. Similar rabbits kept unfed for a further 48 hr in a warm environment released fatty acids and glycerol from their brown adipose tissue at a far greater rate, but the rate of glucose uptake was reduced.

4. On the other hand the brown adipose tissue of rabbits kept unfed in a cold environment (20°C) took up circulating free fatty acids as well as glucose and did not release glycerol. The brown adipose tissue of these rabbits was depleted of fat.

5. The rate of blood flow through brown adipose tissue and the exchange of all three metabolites increased fourfold during noradrenaline infusion in the three groups of rabbits.

6. It is concluded (i) that brown adipose tissue releases significant amounts of fatty acids and glycerol into the circulation and that this contribution is greatly increased with noradrenaline infusion and presumably cold exposure, and (ii) that brown adipose tissue depleted of fat produces heat by drawing free fatty acids as well as glucose from the circulation.