Hyperthermia before exercise has been demonstrated to attenuate subsequent endurance capacity in the heat (Gonzalez-Alonso et al ., JAP 1999). Mental fatigue before exercise has also been reported to impair endurance performance in a temperate environment (Marcora et al., JAP 2009). However, a combined effect of hyperthermia and mental fatigue before exercise on endurance capacity has not been evaluated in the heat. Therefore, this study examined the effects of hyperthermia and mental fatigue before exercise on subsequent endurance capacity and cognitive function during heavy exercise in the heat. Eight male volunteers completed four cycling trials at 80% VO₂max until exhaustion in a climatic chamber (30°C, 50% RH). Participants cycled after: a 90 min seated rest (CON), a 90 min demanding cognitive task to induce mental fatigue (MF), a 30 min water immersion (40°C) to induce hyperthermia following a 60 min seated rest (HT), or a 90 min demanding cognitive task with a 30 min water immersion (40°C) during the last 30 min of task (MF + HT). A demanding cognitive task before exercise and the cognitive function test after exhaustion involved the completion of several computer-based tests which included the Stroop, Sternburg and RVIP tests. Rectal temperature at the start of exercise was higher in HT and MF + HT than in CON and MF (p < 0.0001). Self-reported mental fatigue at the start of exercise was higher in MF and MF + HT than in CON and HT (p < 0.05). Exercise time to exhaustion was less in MF + HT (9 ± 3 min) than in CON (18 ± 7 min) and MF (17 ± 7 min, p < 0.05). At the point of exhaustion, rectal temperature, mean skin temperature, heart rate and cutaneous vascular conductance were not different between trials, but body heat storage was higher in MF + HT than in CON (p < 0.05). In conclusion, hyperthermia with mental fatigue before exercise impairs subsequent endurance capacity during heavy exercise in the heat. This early fatigue is accompanied by higher body heat storage.