Introduction In a hot environment, the progressive rise in heart rate (HR) and fall in stroke volume (SV) during prolonged exercise (cardiovascular drift, CVdrift) is associated with a reduction in maximal oxygen uptake (VO2max) (Wingo et al., 2005). Ten days of heat acclimation (HA) has been shown to increase plasma volume (PV) and maximal cardiac output (Qmax), plus improve VO2max performance in hot and cool conditions during cycling exercise (Lorenzo et al., 2010). This study aimed to determine the effect of 5 days HA on CVdrift and subsequent VO2max performance during running, in hot and cool conditions. We hypothesised that 5 days HA would reduce markers of CVdrift (i.e. HR, SV and Q; thermoregulatory strain [rectal (Tcore)] and perceived exertion (RPE) during running at a standardised workload, thus enhancing VO2max performance in hot and cool conditions compared to PRE-HA. Methods Ten trained middle-distance runners (VO2max>50ml×kg×min-1) will perform a standardised exercise test (running for 30-minutes, 9kph/2% gradient) in hot [40°C, 40% relative humidity (RH)] and cool (15°C, 40% RH) conditions, followed immediately by a VO2max test PRE and POST 5 days consecutive HA (90-min controlled hyperthermia, Tre >38.5°C, in 40°C, RH 55%). Q will be estimated from VO2 via=VO2/arteriovenous O2 content difference from breath-by-breath indirect calorimetry (Vmax Vyntus, Carefusion). Results Preliminary data (n=4) indicates that typical HA adaptations occured, with resting Tcore and HR decreasing from day 1 to 5 of HA (37.2 ±1.1 vs 36.9 ±0.9°C and 54 ±6.2 vs 50 ±4.8 beats×min-1). After 30-min standardised exercise, HR was lower POST-HA compared to PRE-HA in hot conditions but not in cool conditions (157 ±8.01 vs 150 ±6.5 and 128 ±6.5 vs 129 ±4.8 beats×min-1, respectively). HA attenuated the rise in Tcore in hot conditions but not in cool conditions (38.9 ±1.1 vs 38.5 ±0.6°C and 38 ±1.2 vs 38 ±0.9°C, respectively). SV and RPE were reduced in cool and hot conditions POST-HA compared to PRE-HA (Q: 18.9 ±3.7 vs 17.1 ±1.5 L/min and 22.1 ±2.1 vs 21.5 ±2.5 L/min; SV: 153.2 ±13.7 vs 134 ±25.6 mL/min and 153 ±26.7 vs 145 ±26.9 mL/min; RPE: 12 ±0.6 vs 11 ±0.6 and 14 ±1.5 vs 11 ±0.6, respectively). 5-days HA increased VO2max in cool conditions (3%) but not in hot conditions. At VO2max, Q and SV were greater POST-HA compared to PRE-HA in cool and hot conditions (26.43 ±3.2 vs 24.88 ±3.5 L/min and 147.9 ±15.8 vs 134.9 ±22.4 mL/min and 27.17 ±4.2 vs 26.62 ±1.5L/min and 145.5 ±27.2 vs 141.3 ±2.7 mL/min, respectively). Conclusion These preliminary data indicate that 5-days HA reduced some markers of cardiovascular and thermoregulatory strain, whilst improving perceived exertion during exercise heat stress, but these changes had no effect on VO2max in the hot condition. This study is ongoing and a full set of data will be presented at the conference.