Two investigations were designed that (a) evaluated the effect of the respiratory alkalosis that accompanies breathing an hypoxic (H) gas mixture and (b) examined the influence of the duration of breathing this H mixture on the subsequent performance of 45 s supramaximal dynamic exercise. In experiment 1, 12 men performed a 45-s Wingate Test (WT) on three occasions breathing a normoxic (N; 20.9% O2), H (11.3% O2), or normocapnic hypoxic (H+CO2; 11.5% O2, 2.25% CO2) gas mixture for 20 min prior to performing the WT. For experiment 2, nine men performed a 20-min normoxic (N20) and three hypoxic WT trials which consisted of breathing an 11% O2 balance N2 gas mixture for 10 min (H10), 20 min (H20) or 30 min (H30) prior to the WT. For experiment 1, VO2 was significantly reduced during the 45-s H [mean (SD); 1.22 (0.23) l] and H+CO2 [1.12 (0.18) l] trials compared with the N trial [1.78 (0.18) l] Peak power output (Wpeak) during WT was similar across trials. However, a small (less than 3%) but significant reduction in the mean power output (W) was observed in both the H and H+CO2 trials [6.8 (0.6) W.kg-1] compared with the N trial [7.0 (0.6) W.kg-1]. Prior to performing the WT, blood pH and PCO2 were similar [7.40 (0.02) and 5.3 (0.3) kPa, respectively] for the N and H+CO2 trials. A respiratory alkalosis accompanied the H condition [7.46 (0.02) for pH and 4.6 (0.3) kPa for PCO2.(ABSTRACT TRUNCATED AT 250 WORDS)