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Six male rowers rowed maximally for 2500 m in ergometer tests during normoxia (fractional concentration of oxygen in inspired air, F(I)O2 0.209), in hyperoxia (F(I)O2 0.622) and in hypoxia (F(I)O2 0.158) in a randomized single-blind fashion. Oxygen consumption (VO2), force production of strokes as well as integrated electromyographs (iEMG) and mean power frequency (MPF) from seven muscles were measured in 500-m intervals. The iEMG signals from individual muscles were summed to represent overall electrical activity of these muscles (sum-iEMG). Maximal force of a stroke (Fmax) decreased from the 100% pre-exercise maximal value to 67 (SD 12)%, 63 (SD 15)% and 76 (SD 13)% (P < 0.05 to normoxia, ANOVA) and impulse to 78 (SD 4)%, 75 (SD 14)% and 84 (SD 7)% (P < 0.05) in normoxia, hypoxia and hyperoxia, respectively. A strong correlation between Fmax and VO2 was found in normoxia but not in hypoxia and hyperoxia. The mean sum-iEMG tended to be lower (P < 0.05) in hypoxia than in normoxia but hyperoxia had no significant effect on it. In general, F(I)O2 did not affect MPF of individual muscles. In conclusion, it was found that force output during ergometer rowing was impaired during hypoxia and improved during hyperoxia when compared with normoxia. Moreover, the changes in force output were only partly accompanied by changes in muscle electrical activity as sum-iEMG was affected by hypoxic but not by hyperoxic gas. The lack of a significant correlation between Fmax and VO2 during hypoxia and hyperoxia may suggest a partial uncoupling of these processes and the existence of other limiting factors in addition to VO2.