To assess the effect of branched-chain amino acids (BCAA) supplementation on endurance performance in the heat, six women and seven men participated in two trials of rest in the heat (Ta = 34.4 +/- 1.8 degrees C; rh = 39 +/- 14%), followed by 40% VO2peak exercise to exhaustion. Subjects ingested 5 mL x kg(-1) of a placebo (PLAC) or BCAA drink every 30 min. Cycle time to exhaustion increased during BCAA (153.1 +/- 13.3 vs 137.0 +/- 12.2 min, P < 0.05) for men and women. Plasma glucose was maintained at baseline values for both drinks; however, women had significantly higher concentrations (5.9 +/- 0.6 vs 4.0 +/- 0.2 mM, P < 0.05). Plasma free fatty acids and ammonia were not influenced by drink or gender but increased over time. BCAA resulted in a significant (P < 0.05) increase in plasma BCAA (1209 +/- 119 vs 496 +/- 44 microM), while F-TRP (9.6 +/- 0.9 vs 12.0 +/- 1.3 microM) and F-TRP:BCAA were decreased (0.009 +/- 0.001 vs 0.024 +/- 0.003 ND) in both men and women. Cardiovascular and thermoregulatory data were similar between treatments for all subjects. Psychological data were not influenced by BCAA. These results indicate BCAA supplementation prolongs moderate exercise performance in the heat.