The sweating responses to thermal stimulation and to the intradermal injection of acetyl choline or methacholine were measured in 28 men and 18 women aged 70 and over and were compared with the responses in young control subjects of both sexes. There was found to be a marked reduction in the sweating activity of the majority of aged men in comparison with the younger age groups and the body temperature threshold for the onset of sweating was increased. The reduced response and elevated threshold were even more pronounced in aged females. There is considerable variability in response in different subjects and at different bodily sites. Impairment of thermoregulatory function due to diminished or absent sweating is thought to be one of the factors responsible for increased mortality in the elderly population during heat-waves.