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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.

1. The influence of repeatedly raising the body temperature by radiant heat to a level at which acclimatization to heat is normally acquired was investigated in two series of experiments, the first without the subjects sweating, the second with sweating.2. In a second investigation local sweat-gland activity was induced by drug injections on successive days without raising the body temperature.3. These experiments show that the increased sweating capacity characteristic of acclimatization to heat is a result of sweat-gland activity and does not appear to be induced by or to depend on an elevated body temperature.4. Secretory activity results in a loss of glycogen from sweat-gland cells on the first day of heat exposure but not after the glands have been ;trained' by acclimatization to heat.5. The state of acclimatization has no influence on the threshold concentration of acetylcholine required to elicit sweating when injected intradermally.