1. Fifty-two experiments were made with two Cheviot and two Blackface wether sheep in which heat production and heat emission were determined at environmental temperatures of 8, 20 and 32° C. Initially the sheep were closely clipped to within 1–2 mm. of the skin and the fleece was then allowed to grow throughout the experiments.

2. At 32° C. fleece length had no effect on heat production. At 20° C. metabolism was elevated until fleece length exceeded 18 mm. At 8° C. metabolism was elevated until the fleece length exceeded 35–40 mm. No differences were found between the two breeds in their heat production at a particular temperature provided fleece length was identical.

3. The sensible loss of heat divided by the temperature gradient from the rectum to the environment (conductance) was linearly related to the logarithm of fleece length, both at environmental temperatures above and below the critical temperature.

4. No differences between the two breeds or between them and Down Cross sheep were found with respect to their conductances when devoid of fleece. The insulation provided by unit length of fleece was the same in all three breeds and crosses. The fleece of the Blackfaces grew at twice the rate of that of the Cheviots so that at a given time after shearing, the Blackfaces were more resistant to concold.

5. Studies of the losses of heat by the vaporization of water and of skin and fleece surface temperatures also showed no differences between breeds.

6. Analysis of the relation between heat concold, ductance and fleece length suggests that vasoconstriction and vasodilation border on all-or-none effects.