The ergogenic potential of heat is contentious[1]. Long-term (≥ 10 day) heat acclimation (HA) has been shown to be ergogenic under cool ambient conditions with mechanisms underpinning this including increased maximal oxygen uptake, possibly mediated by plasma volume (PV) expansion and an increased maximal cardiac output [2], as well as reduced physiological strain through improved thermoregulation [3]. Recently, short-term (5 day) HA with restricted fluid intake has been shown to augment PV expansion and accelerate HA relative to euhydrated HA[4]; performance improvements in the heat have been documented in trained men following this regime[5]. This study examined the effect of dehydration on long-term HA and exercise performance in a temperate environment. A within-participant, balanced cross-over design with a three-month wash-out period was employed. Each participant completed both control (euhydrated HA [HAEu]) and intervention (HA with permissive dehydration [HADe]) conditions; achieved by completely restricting fluid intake in the isothermic strain HA sessions; a total of 1.75 L of fluid was provided during each control session. All procedures adhered to ethical standards of the Human Tissues Act and the University’s Ethics Committee and with the Declaration of Helsinki. Eight males (Mean[SD] age 21[3] years; maximal oxygen uptake [VO2max]: 55.1[7.1] mL.kg-1.min-1; peak power output [PPO]: 338[46] W; training: 10[3] hours.week-1) undertook a long-term HA programme (Tamb= 40 °C, 50% rh) with a euhydrated Heat Stress Test (HST) (60 mins cycling at 35% PPO, preceding, mid-way though, immediately following and one week after HA (HA consisted of eight isothermic strain sessions: 90 mins.day-1, target rectal temperature [Tre] of 38.5-38.7 °C). A graded exercise test (GXT) for determination of blood lactate threshold (LT), VO2max and PPO was performed in a temperate environment (22 °C, 50% rh) pre- and post-HA. HA induced adaptation when exercising in the heat (Table 1), although there were no differences in the extent of HA between HAEu and HADe, except for the maintenance (Post- to Decay-HA) of a greater Δ blood volume in the HAEu condition (7.0[5.6%]) compared to a return to baseline from Post- to Decay-HA in the HADe condition (P<0.05). HAEu and HADe reduced thermal and cardiovascular strain similarly in a temperate environment following long-term HA and an improvement in oxygen pulse was also observed (P<0.05). Performance trials in a temperate environment suggest that PPO was improved following long term HA but did not differ between conditions and VO2max and LT were unaltered. HA induced favourable thermal, thermoregulatory, physiological and cardiovascular responses to exercise in hot and temperate environments in trained men. However, the rate of HA was not improved with dehydration as an additional stimulus and the ergogenic benefits of HA were unsupported.