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 independent stimulus for adaptation. Methods: Nine aerobically fit males heat acclimated using controlled‐hyperthermia (rectal temperature 38.5°C) 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 35°C, 60% RH). Results: During acclimation bouts, [aldosterone]plasma rose more across DEH than EUH (95%CI for difference between regimes: 40–411 pg ml−1; P = 0.03; n = 5) and was positively related to plasma volume expansion (r = 0.65; P = 0.05), which tended to be larger in DEH (CI: −1 to 10%; P = 0.06; n = 9). In HSTs, resting forearm perfusion increased more in DEH (by 5.9 ml 100 tissue ml−1 min−1: −11.5 to −1.0; P = 0.04) and end‐exercise cardiac frequency fell to a greater extent (by 11 b min−1: −1 to 22; P = 0.05). Hydration‐related effects on other endocrine, cardiovascular, and psychophysical responses to HSTs were unclear. Rectal temperature was unchanged at rest but was 0.3°C lower at end exercise (P < 0.01; interaction: P = 0.52). Conclusions: Short‐term (5‐day) heat acclimation induced effective adaptations, some of which were more pronounced after fluid‐regulatory strain from permissive dehydration, and not attributable to dehydration effects on body temperature. Am. J. Hum. Biol. 26:311–320, 2014. © 2014 Wiley Periodicals, Inc.