PELTONEN, J. E., J. RANTAMÄKI, S. P. T. NIITTYMÄKI, K. SWEINS, J. T. VIITASALO, and H. K. RUSKO. Effects of oxygen fraction in inspired air on rowing performance.Med. Sci. Sports Exerc., Vol. 27, No. 4, pp. 573–579, 1995. The present study examined the effect the oxygen fraction in inspired air (F2O2) on exercise performance and maximum oxygen consumption (VO2max). Six national level male rowers exercised three 2500-m all-out tests on a Concept II rowing ergometer. Each subject performed one test in normoxia (F2O220.9%), one in simulated hyperoxia (F2O262.2%) and one in simulated hypoxia (F2O215.8%) in a randomized single-blind fashion. The mean final rowing time was 2.3 ± 0.9% (P> 0.01; 95% Cl 1.4–3.2) shorter in hyperoxia and 5.3 ± 1.8% (P> 0.01; 95% Cl 3.1–7.5) longer in hypoxia when compared with normoxia. The effect of F2O2on VO2maxexceeded its effect on exercise performance as VO2maxwas 11.1 ± 5.7% greater (P> 0.01; 95% Cl 5.1–17.1) in hyperoxia and 15.5 ± 3.2% smaller in hypoxia (P> 0.01; 95% Cl 12.2–19.0) than in normoxia. Blood lactate concentration and O2consumption per power unit (ml O2·W-1) failed to indicate statistically significant differences in anaerobic metabolism between normoxia and the other two conditions. These data suggest that there are other parameters besides those of energy metabolism that affect exercise performance as F2O2is modified. These possible mechanisms are discussed in this paper.