The purpose of this study was to measure the influence of quercetin supplementation on ratings of perceived exertion in ultramarathon runners competing in the 160-km Western States Endurance Run (WSER). Sixty-three runners were randomized to quercetin (Q) and placebo (P) groups, and under double blinded methods ingested four supplements per day with or without 250 mg quercetin for 3 weeks before the WSER. Thirty-nine of the 63 subjects (quercetin N = 18, placebo N = 21) finished the race. At the completion of exercise ratings of perceived exertion (RPE) were assessed at aid stations located at 40, 90, 125, 150, and 160 km (finish line). The pattern of change in RPE over time was not significantly different between the Q and P groups. Ratings of perceived exertion (RPE) did not significantly increase throughout the race (15.2 +/- 2.9 at 40 km -14.2 +/- 4.0 at 160 km) for both groups combined. Race times were not different between the groups (Q = 26.4 +/- 0.7 h and P = 27.5 +/- 0.6 h). Significant time main effects (p < 0.001) were found for both serum glucose and cortisol throughout the race. Quercetin supplementation for 3 weeks prior to the WSER had no effect on RPE during competitive self-paced ultramarathon running. Ratings of perceived exertion (RPE) did not increase in a linear fashion but instead fluctuated nonmonotonically throughout the self-paced endurance running event.