The hypothesis according to which male common baldness has developed in the human species as a compensation for the growth of a beard in order to achieve heat loss has been tested. In 100 clean-shaven men direct measurement of the area of glabrous skin on the forehead and calvaria was found to be proportional to that of the hairy skin on the lips, cheeks, chin and neck. During light hyperthermia the evaporation rate on the bald scalp was 2 to 3 times higher than on the hairy scalp. Conversely the evaporation rate was practically equal on the foreheads and chins of women and unbearded young men, while in adult clean-shaven bearded men it was 40% less on the chin than the forehead. These results support the hypothesis that male baldness is a thermoregulatory compensation for the growth of a beard in adults.