1. The effect of body fatness on the fasting heat production and energetic efficiency of adult sheep was studied.
2. Energy balance and heat production were determined in adult wether sheep at three fatness levels given a diet of grass hay and maize at four feeding levels, including fasting, in open-circuit respiration chambers. The sheep weighed approximately 60 kg at the start and were fed over a period of 4–7 months to obtain one of three levels of bodyfatness: fat 90 kg, medium 70 kg and thin 55 kg.
3. Neither the digestibility nor the metabolizability of the diet was affected by body fatness or feeding level.
4. Average fasting heat production was 6.47, 7.00 and 8.20 MJ/d for the thin, medium and fat sheep respectively, but when expressed as a multiple of metabolic body size (kg W0.75) remained constant at 0.31 MJ/kg W0.75.
5. At each level of feeding the fat sheep produced most heat, but when heat production was expressed as a multiple of fasting heat production or metabolic body size, the differences due to level of fatness disappeared. Heat increment was not affected by fatness and there was no suggestion that energy utilization was improved by loss of condition.
6. It is concluded that white adipose tissue is highly active and contributes significantly to the maintenance energy requirements of adult sheep. Metabolic body size is a good unit of reference for comparison of energy metabolism in adult animals of the same species but varying in body size.