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HOUMARD, J. A., D. L. COSTILL, J. A. DAVIS, J. B. MITCHELL, D. D. PASCOE, and R. A. ROBERGS. The influence of exercise intensity on heat acclimation in trained subjects. Med. Sci. Sports Exerc., Vol. 22, No. 5, pp. 615–620, 1990. Low-intensity exercise (≤50% V̇O2max) has been demonstrated to produce heat acclimation (HA) in trained subjects. The purpose of this study was to determine whether shorter-duration, moderate-intensity exercise would also result in HA. Nine trained runners performed two 9-d exercise heat-stress protocols. Each protocol consisted of a 90-min heat tolerance test on days 1 (HTT1) and 9 (HTT2). On days 2–8 the subjects exercised at 50% V̇O2max for 60 min·d−1 (T50) or at 75% V̇O2max for 30–35 min·d−1 (T75). Final HTT2 heart rate and rectal temperature (Tr) were significantly (P < 0.001) reduced, as compared to HTT1, with no differences between T50 and T75. Both protocols resulted in significant (P < 0.05) reductions in HTT2 pre-exercise Tr and total exercising caloric expenditure, both of which are known to contribute to HA. No changes in resting plasma volume, osmolality, protein, post-HTT aldosterone, and exercising sweat rate were observed. These results demonstrate that equal levels of HA were obtained with T50 and T75, which suggests that moderate-intensity, short-duration exercise in the heat can produce HA in trained subjects.