Most heat acclimation data are from regimes longer than 1 week, and acclimation advice is to prevent dehydration. Objectives: We hypothesized that (i) short-term (5-day) heat acclimation would substantially improve physiological strain and exercise tolerance under heat stress, and (ii) dehydration would provide a thermally independ- ent stimulus for adaptation. Methods: Nine aerobically fit males heat acclimated using controlled-hyperthermia (rectal temperature 38.5C) for 90 min on 5 days; once euhydrated (EUH) and once dehydrated (DEH) during acclimation bouts. Exercising heat stress tests (HSTs) were completed before and after acclimations (90-min cycling in Ta 35C, 60% RH). Results: During acclimation bouts, (aldosterone)plasma rose more across DEH than EUH (95%CI for difference between regimes: 40-411 pg ml 21 ; P 50.03; n 55) and was positively related to plasma volume expansion (r 50.65; P 50.05), which tended to be larger in DEH (CI: 21 to 10%; P 50.06; n 59). In HSTs, resting forearm perfusion increased more in DEH (by 5.9 ml 100 tissue ml 21 min 21 : 211.5 to 21.0; P 50.04) and end-exercise cardiac frequency fell to a greater extent (by 11 b min 21 : 21 to 22; P 50.05). Hydration-related effects on other endocrine, cardiovascular, and psychophysical responses to HSTs were unclear. Rectal temperature was unchanged at rest but was 0.3C lower at end exercise (P