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Purpose: 7 days L-citrulline supplementation has been reported to improve blood pressure, V . O 2 kinetics, gastrointestinal (GI) perfusion and endurance cycling performance through increasing arterial blood flow. In situations where blood volume is compromised (e.g., hyperthermia/hypohydration), L-citrulline may improve thermoregulation and exercise performance by redistributing blood flow to aid heat loss and/or muscle function. This study assessed 7 days L-citrulline supplementation on running performance in the heat, whilst mildly hypohydrated., Methods: 13 endurance runners (2 female, 31 ± 8 y, V . O 2 peak 60 ± 6 mL/kg/min) participated in a randomised crossover study with 7 days L-citrulline (CIT; 6 g/d) or placebo (maltodextrin powder; PLA) supplementation. Participants completed a 50 min running 'preload' at 65% V . O 2 peak (32 °C, 50% relative humidity) to induce hyperthermia and hypohydration before a 3 km running time trial (TT). Body mass and blood samples were collected at baseline, pre-preload, post-preload and post-TT, whilst core and skin temperature, heart rate and perceptual responses were collected periodically throughout., Results: TT performance was not different between trials (CIT 865 ± 142 s; PLA 892 ± 154 s; P = 0.437). Core and skin temperature and heart rate (P ≥ 0.270), hydration (sweat rate, plasma volume, osmolality) indices (P ≥ 0.216), GI damage (P ≥ 0.260) and perceptual responses (P ≥ 0.610) were not different between trials during the preload and TT., Conclusions: 7 days of L-citrulline supplementation had no effect on 3 km running performance in the heat or any effects on thermoregulation or GI damage in trained runners in a hypohydrated state., Competing Interests: Declarations. Conflict of interest: This is a summary of independent research funded by Loughborough University and carried out at the National Institute for Health and Care Research (NIHR) Leicester Biomedical Research Centre (BRC). The views expressed are those of the author(s) and not necessarily those of Loughborough University, the NIHR or the Department of Health and Social Care. M.P.F. is funded by the National Institute for Health Research (NIHR) Applied Research Collaboration East Midlands (ARC EM). S.J.B. has current/previous funding from KIRIN Central Research Institute, Fitoplancton Marino SL, Kaneka Medical Europe, Bio-gen Extracts and Herbalife Europe Ltd. In all cases, monies have been paid to S.J.B’s institution and not directly to SJB. S.A.M. has current/previous funding from Entrinsic Beverage Company LLP and Herbalife Europe Ltd and has received honorariums from Unilever PLC to present at conferences. L.J.J. has current/previous funding from Bunge Loders Croklaan B.V., Herbalife Europe Ltd, Bridge Farm Nurseries ltd., Entrinsic Beverage Company LLP, Decathlon SA, PepsiCo Inc. and Volac International, has performed consultancy for The British Horseracing Authority, Lucozade, Ribena Suntory and PepsiCo Inc., and has received conference fees from PepsiCo Inc. and Danone Nutricia. In all cases, monies have been paid to LJJ’s institution. LJJ is part of the National Institute for Health Research’s Leicester Biomedical Research Centre, which is a partnership between University Hospitals of Leicester NHS Trust, Loughborough University, and the University of Leicester., (© 2024. The Author(s).)