Two studies were conducted to investigate the effects of induced metabolic alkalosis during training (via NaHCO3 ingestion) on changes in MCT, NBC, bm, endurance performance and mitochondrial respiration. In the first study, 2 groups of 8 females trained with high-intensity intervals (2 min@110%VO2max: 1 min rest), three times per week, for eight weeks. One group ingested NaHCO3 (0.4 g/kg) while the control group ingested a placebo (NaCl, 0.2 g/kg) prior to each training session. In the second study, 24 male Wistar rats basically replicated the same study. Contrary to our hypothesis, in both studies, ingesting NaHCO3, and therefore altering the likely accumulation of H+ during training, did not affect the changes seen in bm, MCT1 or NBC. There was however, a significantly greater increase in MCT4 in the soleus muscle of the rats who ingested NaHCO3 before training. In both the humans and the rats, there was a significantly greater increase in endurance performance in the NaHCO3 group. It is hypothesised that this may have been due to a reduced metabolic acidosis during training resulting in greater improvements in muscle oxidative capacity as there was also a significantly greater increase in state 3 respiration in the soleus muscle of the rat NaHCO3 group. Our results suggest that metabolic acidosis during training may impair some chronic adaptations to training.