Purpose This study determined the effects of pre-exercise sodium bicarbonate ingestion (ALK) on changes in oxygen uptake (V˙ O2) at the end of a supramaximal exercise test (SXT). Methods Eleven well-trained cyclists completed a 70-s all-out cycling effort, in double-blind trials, after oral ingestion of either 0.3 g kg−1 of sodium bicarbonate (NaHCO3) or 0.2 g kg−1 body mass of calcium carbonate (PLA). Blood samples were taken to assess changes in acid–base balance before the start of the supramaximal exercise, and 0, 5 and 8 min after the exercise; ventilatory parameters were also measured at rest and during the SXT. Results At the end of the PLA trial, which induced mild acidosis (blood pH = 7.20), subjects presented a significant decrease in V˙ O2 (P < 0.05), which was related to the amplitude of the decrease in minute ventilation (˙VE) during the SXT (r = 0.70, P < 0.01, n = 11). Pre-exercise metabolic alkalosis significantly prevented the exercise-induced decrease in V˙ O2 in eleven well-trained participants (PLA: 12.5 ± 2.1 % and ALK: 4.9 ± 0.9 %, P < 0.05) and the decrease in mean power output was significantly less pronounced in ALK (P < 0.05). Changes in the V˙ O2 decrease between PLA and ALK trials were positively related to changes in the ˙VE decrease (r = 0.74, P < 0.001), but not to changes in power output (P > 0.05). Conclusions Pre-exercise alkalosis counteracted the V˙ O2 decrease related to mild acidosis, potentially as a result of changes in ˙V E and in muscle acid–base status during the all-out supramaximal exercise.