&p.1:Abstract Acute and repeated exposure for 8–13 consec- utive days to exercise in humid heat was studied. Twelve fit subjects exercised at 150 W [45% of maximum O 2 uptake (V . O 2 , max )] in ambient conditions of 35°C and 87% relative humidity which resulted in exhaustion after 45 min. Average core temperature reached 39.9 ± 0.1°C, mean skin temperature (T – sk ) was 37.9 ± 0.1°C and heart rate (HR) 152 ± 6 beats min –1 at this stage. No effect of the increasing core temperature was seen on cardiac out- put and leg blood flow (LBF) during acute heat stress. LBF was 5.2 ± 0.3 l min –1 at 10 min and 5.3 ± 0.4 l min –1 at exhaustion (n = 6). After acclimation the sub- jects reached exhaustion after 52 min with a core tem- perature of 39.9 ± 0.1°C, T – sk 37.7 ± 0.2°C, HR 146 ± 4 beats min –1 . Acclimation induced physiological adapta- tions, as shown by an increased resting plasma volume (3918 ± 168 to 4256 ± 270 ml), the lower exercise heart rate at exhaustion, a 26% increase in sweating rate, lower sweat sodium concentration and a 6% reduction in exer- cise V . O 2 . Neither in acute exposure nor after acclimation did the rise of core temperature to near 40°C affect me- tabolism and substrate utilization. The physiological ad- aptations were similar to those induced by dry heat accli- mation. However, in humid heat the effect of acclimation on performance was small due to physical limitations for evaporative heat loss. &kwd:Key words Core temperature · Cardiovascular system · Hormones · Metabolic responses · Heat stress&bdy: Introduction In hot, dry environments, in which exercise performance is limited by hyperthermia, the key factors for perfor- mance improvement are an increased sweating rate to optimize the core-to-skin gradient and an expansion of the blood volume. These adaptations induced by repeat- ed exposure to exercise in dry heat allow for better main- tenance of stroke volume and a lowering of the heart rate. In humid heat, in which the rate of sweating ex- ceeds that of evaporation already when unacclimatized, the question arises as to whether any of the well-de- scribed physiological adjustments to chronic heat expo- sure occur. There is some support in the literature for the lack of an effect. In studies by Wyndham et al. [42, 43] only a minor increase in cardiac output during exercise was observed after eight days of exposure to a hot wet environment. Moreover, early signs of adaptation have been shown to be transient, vanishing after a week de- spite continuous heat exposure [12, 36, 38, 42, 43]. One of the limitations in previous studies is that the heat strain has not been equalized, i.e., external and in- ternal heat loads in the dry and wet hot environments have not been the same. Furthermore, only few have studied both dry and humid acclimation with the same procedures. A direct comparison of results from earlier studies performed in dry and humid heat conditions is therefore difficult. In more recent studies the wet bulb globe temperature (WBGT index, [18]) has been used to equalize stress levels [15, 16, 37]. However, as judged by the acute physiological response the strain was neverthe- less different with the humid heat being more stressful than dry heat despite similar WBGTs. In the present study another approach has been taken to equalize heat loads. In an earlier study in dry heat the metabolic (internal) and environmental (external) heat loads caused the core temperature T – core to increase by 0.066°C min –1 , and exhaustion was reached within 40–45 min at the first exposure with a T – core very close to 40°C [26]. Based on this previous study [26] our hypoth- esis was that the rate of rise in T – core and the high, final B. Nielsen ( ✉ ) · S. Strange · B. Saltin August Krogh Institutet, Universitetsparken 13, DK-2100 Copenhagen Ø, Denmark N.J. Christensen Department of Endocrinology, Herlev Hospital, Copenhagen, Denmark J. Warberg Department of Medical Physiology, The Panum Institute, University of Copenhagen, Copenhagen, Denmark&/fn-block: Pflügers Arch – Eur J Physiol (1997) 434:49–56 © Springer-Verlag 1997 ORIGINAL ARTICLE &roles:Bodil Nielsen · Søren Strange · Niels Juel Christensen Jørgen Warberg · Bengt Saltin Acute and adaptive responses in humans to exercise in a warm, humid environment &misc:Received: 3 July 1996 / Received after revision: 26 September 1996 / Accepted: 7 January 1997