&p.1:Abstract The purpose of this investigation was to ex- amine the effects of aging and aerobic fitness on exer- cise- and methylcholine-induced sweating responses dur- ing heat acclimation. Five younger [Y group – age: 23±1 (SEM) years; maximal oxygen consumption (V . O 2max ): 47±3 ml·kg –1 ·min –1 ], four highly fit older (HO group – 63±3 years; 48±4 ml·kg –1 ·min –1 ) and five normally fit older men (NO group – 67±3 years; 30±1 ml·kg –1 ·min –1 ) who were matched for height, body mass and percentage fat, were heat acclimated by daily cycle exercise (≈35% V . O 2max for 90 min) in a hot (43°C, 30% RH) environ- ment for 8 days. The heat acclimation regimen increased performance time, lowered final rectal temperature (T re ) and percentage maximal heart rate (%HR max ), improved thermal comfort and decreased sweat sodium concentra- tion similarly in all groups. Although total body sweat- ing rates (M . sw ) during acclimation were significantly greater in the Y and HO groups than in the NO group (P<0.01) (because of the lower absolute workload in the NO group), the M . sw did not change in all groups with the acclimation sessions. Neither were local sweating rates (m . sw ) on chest, back, forearm and thigh changed in all groups by the acclimation. The HO group presented greater forearm m . sw (30–90 min) values and the Y group had greater back and thigh m . sw (early in exercise) values, compared to the other groups (P<0.001). In a methylcho- line injection test on days immediately before and after the acclimation, the order of sweat output per gland (SGO) on chest, back and thigh was Y>HO>NO, and on the forearm Y=HO>NO. No group differences were ob- served for activated sweat gland density at any site. The SGO at the respective sites increased in the post-accli- mation test regardless of group (P<0.01), but on the thigh the magnitude of the increase was lower in the NO (P<0.02) and HO (P=0.07) groups than in the Y group. These findings suggest that heat tolerance and the im- provement with acclimation are little impaired not only in highly fit older but also normally fit older men, when the subjects exercised at the same relative exercise inten- sity. Furthermore, the changes induced by acclimation appear associated with an age-related decrease in V . O 2max . However methylcholine-activated SGO and the magnitude of improvement of SGO with acclimation are related not only to V . O 2max but also to aging, suggesting that sensitivity to cholinergic stimulation decreases with aging. &kwd:Key words Aging · Thermoregulation · Cholinergic stimulation · Sweat gland output&bdy: Introduction The main effector response for dissipating heat when ex- posed to environmental heat is sweating. Many have re- ported that sweating declines with increasing age (Wag- ner et al. 1972; Fennel and Moore 1973; Anderson and Kenney 1987; Kenney and Fowler 1988; Sato and Timm 1988; Tankersley et al. 1991; Armstrong and Kenney 1993; Inoue et al. 1991, 1995; Inoue 1996; Inoue and Shibasaki 1996). We have suggested that the lower sweat gland output with aging may reflect age-related changes in the sweat gland itself (atrophy of some sweat glands) and/or in its pharmacological sensitivity (Anderson and Kenney 1987; Kenney and Fowler 1988; Inoue et al. 1991; Inoue 1996; Inoue and Shibasaki 1996). Several investigators (Drinkwater et al. 1982; Pandolf et al. 1988; Smolander et al. 1990; Havenith et al. 1995) have suggested that aging per se has no effect on sweating Y. Inoue ( ✉ ) Laboratory for Human Performance Research, Osaka International University for Women, 6-21-57 Tohda-Cho, Moriguchi, Osaka 570-8555, Japan e-mail: inoueyos@oiuw.oiu.ac.jp, Tel.: +81-6-902-0791 ext. 363, Fax: +81-6-902-8894 G. Havenith Department of Human Sciences, Loughborough University, Loughborough, Leics LE11 3TU, UK W.L. Kenney · J.L. Loomis · E.R. Buskirk Noll Physiological Research Center, The Pennsylvania State University, University Park, PA-16802, USA&/fn-block: Int J Biometeorol (1999) 42:210–216 © ISB 1999 ORIGINAL ARTICLE &roles:Yoshimitsu Inoue · George Havenith W. Larry Kenney · Joseph L. Loomis Elsworth R. Buskirk Exercise- and methylcholine-induced sweating responses in older and younger men: effect of heat acclimation and aerobic fitness &misc:Received: 8 May 1998/Accepted: 5 October 1998