1. Thirty wether sheep comprising five of each of the following breeds, Scottish Blackface, Welsh Mountain, Cheviot, Suffolk Down, Kent and Hampshire Down, were subjected to a standard series of experiments in which energy metabolism was measured during fasting and at the maintenance level of nutrition, and in which maximal voluntary intake of food was measured and the metabolic responses to the stress of wind (10 miles/h) and rain (1 cm/h) were determined. 2. The weight of an individual sheep at the maintenance level of feeding was 8.4% greater and when given feed ad lib. 22% greater than its weight when fasted. These increases largely reflected changes in the weight of gut contents. 3. When fasting metabolism was expressed per kg weight raised to the power 0.73, the small Welsh Mountain sheep had the lowest metabolism of 54.1 kcal/kg W0.73 and the Cheviot sheep the highest of 64.4 kcal/kg W0.73. Overall breed differences were statistically significant (0.05 > P > 0.01). Evidence collected in the experiments, however, suggests that fasting metabolism was more closely related to body-weight raised to the power 0.85. When this basis for breed comparison was used, differences in fasting metabolism between breed groups disappeared. 4. No differences between breed groups in the proportion of the energy they ingested which was lost in faeces, in urine or as methane, were found when they were given food at a maintenance level. The Welsh Mountain sheep, however, had the smallest heat production at the maintenance level when expressed as kcal/kg W0.73. 5. No differences between breed groups in the apparent digestibility of the energy of feed given ad lib. were found. The voluntary intake increased with weight of sheep. When the amount of feed energy consumed and the energy apparently digested were related to the determined maintenance requirement for apparently digested energy of each sheep, no differences between breed groups were found. The efficiency of feed utilization by these breed groups of sheep when given feed ad lib. was the same. 6. It was found that the Hampshire sheep were the most resistant to the effect of wind on their heat production, and the Welsh Mountain sheep the least. The Scottish Blackface was most resistant to the effect of rain on heat production. 7. It is concluded that metabolic differences between different breeds of sheep differing widely in size are quite small, but that breeds differ markedly in their resistance to environmental stresses caused by wind and rain, and that these largely reflect the characteristic fleece types of the breeds concerned.