Nutritional adenosine antagonists can enhance endurance exercise performance in temperate environments, but their efficacy during heat stress is not well understood. This double-blinded, placebo controlled study compared the effects of an acute dose of caffeine or quercetin on endurance exercise performance during compensable heat stress (40ºC, 20-30%rh). On each of three occasions, ten healthy men performed 30-minutes of cycle ergometry at 50% VO_2peak followed by a 15-min performance time trial after receiving either (P) placebo, (C) caffeine (9 mg/kg), or (Q) quercetin (2,000 mg). Serial blood samples, physiological (heart rate, rectal, and mean skin body temperatures), perceptual (ratings of perceived exertion, pain, thermal comfort, motivation), and exercise performance measures (total work and pacing strategy) were made. Supplementation with C and Q increased pre-exercise blood concentrations of caffeine (55.62 ± 4.77 µM) and quercetin (4.76 ± 2.56 µM) above their in vitro inhibition constants for adenosine receptors. No treatment effects were observed for any physiological or perceptual measures, with the exception of elevated rectal body temperatures (0.20 - 0.30°C; P<0.05) during C versus Q and P. Supplementation did not affect total work performed (P: 153.5 ± 28.3; C: 157.3 ± 28.9; Q: 151.1 ± 31.6 kJ; P>0.05) or the self-selected pacing strategy employed. These findings indicate that the nutritional adenosine antagonists caffeine and quercetin do not enhance endurance exercise performance during compensable heat stress.