1. The rectal temperatures of three 4-month-old Ayrshire bull calves were measured in individual 6 hr. daily exposures to 15, 20, 25, 30, 35 and 40° C. dry-bulb temperature at low humidity (17 mg./l. absolute humidity) and to 30, 35 and 40° C. at high humidity (7 mg./l. saturation deficit). The experiments on each animal lasted 45 consecutive days and consisted of five replications of these nine environmental conditions. Measurements of rectal temperature were made every 5 min.2. The rectal temperatures of the animals rose with increasing environmental temperature and humidity.

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1. An investigation has been made of the structure, distribution and dimensions of the so-called sweat glands in twenty-one different body regions of each of five 3- to 4-year-old Ayrshire cows, and incidental studies have been made on calves and embryos.2. It has been observed that in all those regions each hair follicle is accompanied by an arrector pili muscle, a sweat gland and a sebaceous gland. This combination has been designated a ‘hair follicle unit’.3. In the skin of embryos the sweat gland appears as a single unbranched tube and in the skin of calves and cows it is a bag-shaped gland with a long slender duct which opens on the skin surface as a funnel-shaped outlet.4. The gland is composed of two layers of cells, an outer myoepithelium and inner glandular epi thelium.5. What may be successive stages in the intraluminal transformation of the secretory products discharged by the gland cells lining the lumen, are illustrated by a series of photographs.6. The glands have a poor blood supply and appear to be apocrine. It is unlikely, therefore, that the sweat glands of Ayrshire cows function as effectively in heat regulation as human sweat glands.7. The number of sweat glands per sq.cm. of skin was measured in all the regions. The average number was 1871 per sq.cm., ranging from about 1000 in the lower limbs to about 2500 in the axilla and neck regions.8. The length and circumference of the gland were measured, hence its secretory surface was calculated. The average area of secretory surface of a single sweat gland was 0·22 sq.mm., while the average area of secretory surface of the sweat glands per sq.cm. of skin was 3·94 sq.cm. There was a significant negative correlation between the number of glands per sq.cm. and the area of secretory surface of a single sweat gland.9. The ventral region of the neck, the axilla and upper hindleg had the largest area of secretory surface per sq.cm. of skin, while the forehead, back (sacral), gluteus, lower foreleg and hindleg had the smallest.10. The only region which had a significantly smaller area of secreting surface than the front flank region was the sacral region of the back.

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