Quercetin is a naturally occurring flavonoid with anti-oxidant and anti-inflammatory properties. The effect of quercetin supplementation on maximal oxygen uptake (VO(2max)) is unknown. The purpose of this investigation was to test the effects of quercetin supplementation on VO(2max) in untrained, sedentary individuals. After baseline treadmill VO(2max) testing, 11 participants (5 males, 6 females) ingested either placebo or quercetin-supplemented (1000 mg x day(-1)) food bars in a randomized, double-blind, counterbalanced, crossover research design. The participants ingested food bars for six consecutive mornings (5 days). On the sixth morning, participants underwent repeat VO(2max) testing. After a 22 day wash-out, the participants repeated baseline VO(2max) testing, daily consumption of the opposite food bars, and post-supplementation VO(2max) testing. The condition x time interaction for VO(2max) was non-significant when expressed in absolute (litres x min(-1); P = 0.929) and relative (ml x kg(-1) x min(-1); P = 0.778) terms. These findings were similar when taking sex into account (P > 0.05). The mean difference in VO(2max) change from pre to post between groups (quercetin vs. placebo) was 0.139 ml x kg(-1) x min(-1) (P = 0.780). Other physiological measures also were similar between conditions (P > 0.05). In conclusion, 5 days of quercetin supplementation did not influence VO(2max) or related variables in sedentary men and women.