Acetazolamide (Acz) is used to prevent acute mountain sickness at altitude. Because it could affect O₂ transport in several different and potentially conflicting ways, we examined its effects on pulmonary and muscle gas exchange and acid base status during cycle exercise at ~30, 50 & 90% VO₂max in normoxia (FIO₂ = 0.2093) and acute hypoxia (FIO₂ = 0.125). In a double-blind, order-balanced, cross-over design, 6 healthy, trained men [normoxic VO₂max = 59 ml • kg^-1 • min^-1] exercised in both FIO₂'s after Acz (3 doses of 250 mg, 8 h apart) and placebo. One week later this protocol was repeated using the other drug (placebo or Acz). We measured cardiac output (QT), leg blood flow (LBF), muscle and pulmonary gas exchange, the latter using the multiple inert gas elimination technique. Acz did not significantly affect VO₂, QT, LBF or muscle gas exchange. As expected, Acz led to lower arterial and venous blood [HCO3^-], pH and lactate levels (P<0.05), and increased ventilation (P<0.05). In both normoxia and hypoxia, Acz resulted in higher arterial PO₂ and saturation and a lower alveolar-arterial PO₂ difference (AaDO₂) due to both less VA/Q mismatch and less diffusion limitation (P<0.05). In summary, Acz improved arterial oxygenation during exercise, due to both greater ventilation and more efficient pulmonary gas exchange. However, muscle gas exchange was unaffected.