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Wrist flexion, at a contraction rate of 0.5 Hz, was performed for 9 min at moderate- (75% of onset of acidosis; intracellular pH threshold) and heavy-intensity (125% intracellular pH threshold) exercise.<jats:sup>31<\/jats:sup>P-magnetic resonance spectroscopy was used to measure intracellular [H<jats:sup>+<\/jats:sup>], [PCr], [P<jats:sub>i<\/jats:sub>], and [ATP]. The initial recovery data were used to estimate the rate of ATP synthesis and oxidative flux at the end of heavy-intensity exercise. In repeated trials, venous blood sampling was used to measure plasma [H<jats:sup>+<\/jats:sup>], [HCO<jats:sub>3<\/jats:sub><jats:sup>\u2212<\/jats:sup>], and [Lac<jats:sup>\u2212<\/jats:sup>]. Throughout rest and exercise, plasma [H<jats:sup>+<\/jats:sup>] was lower ( P &lt; 0.05) and [HCO<jats:sub>3<\/jats:sub><jats:sup>\u2212<\/jats:sup>] was elevated ( P &lt; 0.05) in Alk compared with control. During the final 3 min of heavy-intensity exercise, Alk caused a lower ( P &lt; 0.05) intracellular [H<jats:sup>+<\/jats:sup>] [246 (SD 117) vs. 291 nmol\/l (SD 129)], a greater ( P &lt; 0.05) [PCr] [12.7 (SD 7.0) vs. 9.9 mmol\/l (SD 6.0)], and a reduced accumulation of [ADP] [0.065 (SD 0.031) vs. 0.098 mmol\/l (SD 0.059)]. Oxidative flux was similar ( P &gt; 0.05) in the conditions at the end of heavy-intensity exercise. In conclusion, our results are consistent with a reduced intracellular acidosis, causing a decrease in the magnitude of the PCr slow component. 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