Exercise in the heat has detrimental effects on circulation that might negatively influence endurance performance. If blood is diverted away from the inspiratory muscles to the skin during exercise in the heat, exercise-induced inspiratory muscle fatigue might be exacerbated. Thus, we hypothesised that prolonged heavy endurance exercise in the heat would impair exercise performance and exacerbate inspiratory muscle fatigue compared to exercise in a thermo-neutral environment. Using a crossover design, seven male endurance trained subjects [mean (SEM) maximum oxygen uptake = 62.2 (1.5) ml x kg(-1) x min(-1)] were assigned at random to either a group that exercised in the heat at an ambient temperature of 35 degrees C (H) or a group that exercised in the cool at 15 degrees C (C). Maximum inspiratory mouth pressure at zero flow (P(0)), pressure normalised maximum relaxation rate (MRR/ P(0)), time constant for the pressure decay (tau), and maximum inspiratory flow at 30% P(0) (V(30)) were assessed immediately before and reassessed within 2, 30, and 60 min of completing a pre-loaded time trial [40 min at 65% peak power, plus approximately 30 min time trial] on a cycle ergometer. Group H completed the time trial 432 (135) s slower than group C [2285 (180) vs 1852 (122) s, respectively; Delta=24 (8)%, P=0.0094]. Repeat measurements within 2 min post-exercise revealed significant declines in P(0), MRR/ P(0), tau, and V(30) from baseline values, but no between-group differences were observed. In conclusion, heavy sustained exercise in the heat impaired subsequent time-trial performance but did not exacerbate inspiratory muscle fatigue in endurance-trained subjects.