During prolonged exercise at a given relative intensity (i.e. % of VO2max), endurance-trained individuals (Tr) achieve core temperatures similar to untrained individuals (Untr) despite of higher heat production.1 Tr have enhanced cutaneous blood flow and sweat rate2 allowing greater heat dissipation. However, this phenomenon only has been investigated in a neutral environment (19-24°C) where heat dissipation is not limited. PURPOSE: To investigate if Tr have similar rectal temperatures than Untr when exercising at similar %VO2max in a hot environment that limits heat dissipation. METHODS: Ten endurance-trained (60±6 ml·kg-l·min-1) and ten untrained men (44±3 ml·kg-l·min-1) pedalled during 45 min in a hot-dry environment (36°C; 23 % rh; 2.5 m·s-1 wind speed) on three different occasions at 40, 60, and 80% of their VO2max. Oxygen uptake, rectal temperature, forearm skin blood flow, whole body sweat rate and heart rate were monitored. Two-way ANO VA was used to detect differences. RESULTS: The results are shown in the next table, values are the mean ± SEM.TableCONCLUSION: During exercise in the heat, trained individuals reached similar rectal temperatures and heart rate than untrained counterparts at 40–60% VO2max despite higher heat production. However, during 80%VO2max trained individuals reached a higher rectal temperature than untrained despite the higher skin blood flow and trend for a higher sweat rate. This finding suggests that endurance-trained subjects are more prone to overheating than untrained when exercising intensely in a hot environment. Supported by a grant from Gatorade Sports Science Institute.