INTRODUCTION: Athletic performance may be compromised in extreme heat; this was relevant for the Athens and Beijing Olympics and may be for the 2014 and 2022 FIFA World cups in Brazil and Qatar respectively; under those conditions prior determination of required fluid intake is vital for athlete safety. Thompson(1954) described lower sweating rates in Bantus than Caucasians.AIMS: 1.To determine the English Football Premiership ethnic mix. 2.To establish fluid intake, sweat production and sweat evaporation during exercise in extreme heat and their ethnic differences. 3.To establish cardiorespiratory and other physiological factors contributing to any putative ethnic differencesMETHODS: Using Scout 7 software, the English Football Premiership was analysed for ethnic background in 2012. 34% players were of African origin (AO) and 66% of European origin (EO). On two successive years, studies were performed in Dubai after four days acclimation, during 80 minutes training at 40±2 C ambient temperature; 25±8% relative humidity on 29 professional Premiership footballers from a single team with Ethics clearance. 15 were AO and14 EO. Players could drink ad libitum.RESULTS: There was no difference between AO and EO in height, weight, body surface area, skin fold thickness, resting or average heart rate during exercise, QT interval, cardiac chamber dimensions, left ventricular ejection fraction or maximum VO2. During exercise, fluid intake in AO ranged from 1.1- 3.0l (1.6±0.63l, median±SD) and in EO ranged from 0.5-2.4 l (1.35±0.50 ; p=0.40; unpaired t-test). Sweat production in AO ranged from 1.8 – 3.9 l (2.3±0.54 l) higher than 1.3 – 2.7 l in EO (2.00±0.43 l; p = 0.03). Mean±SD of Peak Heart Rate (Polar Electro) during training in the heat was 186±9 bpm (range 172-200 bpm) in AO footballers, significantly higher than EO players (178±9 bpm, range 163-187 bpm; p<0.02)Heart rate variability (HRV) was calculated from a five minute collection of resting ECG using bespoke frequency domain analysis software using American Heart Association guidelines (Omega Wave; Oregon USA; Akselrod et al, 1981). (Mean±SD) LF/HF ratio, an index of sympatho-vagal tone balance was 1.07±0.48 in AO and significantly higher in EO 2.0±1.27 (p= 0.03, Fig 1). CONCLUSIONS: 1. A large proportion of the English Premiership footballers are of African origin2. Sweat production during exercise in extreme heat may be up to 3.25 L/hr, and fluid intake 2L/hr production. This can lead to significant dehydration of up to 2.4% naked body mass.3. There appears to be increased sweat production in AO players, despite a similar fluid intake to EO players; this may confer a performance advantage to exercising in the heat.4. Increased sweat production in AO players appears associated with an increased sympathetic tone, which has also been related to an increased risk of cardiac events (Tsuji et al, 1996)