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1. Sweat output has been measured continuously from four Welsh Mountain sheep exposed on several occasions for periods of 8 hr to air temperatures of 20, 30 and 40° C. At all temperatures sweat was discharged intermittently. Discharges occurred more frequently at high air temperatures and when the animals were shorn, but not at consistently different frequencies in summer and winter.

2. At 40° C T_a, sweat discharges were initially large, decreased in size during the first 1-3 hr, and then continued at regular intervals and at an approximately constant size for the remainder of the experiments. Most, and probably all, sweat glands in the skin were active at each discharge.

3. The volumes of sweat glands, determined histologically, decreased after prolonged activity in the heat. Comparison of the changes in sweat gland volumes with the amounts of sweat discharged led to conclusions that the decline in sweat output seen on initial exposure to heat was probably due to depletion of secretory fluid from storage within the gland and that the subsequent regular discharge of small amounts of sweat was due to expulsion of newly formed secretion.

4. Examination of serial sections of midside skin revealed that in these sheep sweat glands consisted of a superficial non-convoluted duct separated from a large ampulla by a region in which the gland was convoluted. After long periods of sweating, the glands decreased in diameter in both the convoluted and non-convoluted regions but were never found to be empty.

5. The pattern of sweat output from sheep during prolonged exposure to heat appears to be determined not only by processes of secretion and intermittent expulsion but also by the amount and rate of depletion of preformed sweat stored in the gland lumen.