This study examined the acute and chronic effects of euhydrated and hypohydrated heat exposure, on biomarkers of stress and inflammation. Eight trained males [mean (SD) age: 21 (3) y; mass: 77.30 (4.88)kg; VO2max: 56.9 (7.2)mLkg(-1)min(-1)] undertook two heat acclimation programmes (balanced cross-over design), once drinking to maintain euhydration and once with restricted fluid-intake (permissive dehydration). Days 1, 6, and 11 were 60min euhydrated exercise-heat stress tests (40 degrees C; 50% RH, 35% peak power output), days 2-5 and 7-10 were 90min, isothermal-strain (target rectal temperature: 38.5 degrees C) exercise-heat sessions. Plasma was obtained pre- and post- exercise on day 1, 2, and 11 and analysed for cortisol, interleukin-6 (IL-6), and C-reactive protein (CRP). Cortisol and CRP were also assessed on day 6. IL-6 was elevated following the initial (acute) 90min isothermal heat strain exercise-heat exposure (day 2) with permissive dehydration ((pre exercise: 1.0pgmL(-1) [0.9], post-exercise: 1.8pgmL(-1) [1.0], P=.032) and when euhydrated (pre-exercise: 1.0pgmL(-1) [1.4], post-exercise: 1.6pgmL(-1) [2.1], P=.048). Plasma cortisol levels were also elevated but only during permissive dehydration (P=.032). Body mass loss was strongly correlated with Deltacortisol (r=-0.688, P=.003). Although there was a trend for post-exercise cortisol to be decreased following both heat acclimation programmes (chronic effects), there were no within or between intervention differences in IL-6 or CRP. In conclusion, acute exercise in the heat increased IL-6 and cortisol only when fluid-intake is restricted. There were no chronic effects of either intervention on biomarkers of inflammation as evidenced by IL-6 and CRP returning to basal level at the end of heat acclimation.