High-intensity interval training (HIIT) has often been found to enhance muscle pH regulation through increases in muscle buffer capacity (1) and H+ transport proteins (2). During exercise the largest portion of H+ regulation comes from the monocarboxylate transporters (MCT1 and MCT4) (3). The signalling factors responsible for upregulation of H+ transport proteins are uncertain (4), although H+ accumulation or H+ efflux may be important stimuli (5). Maximising high non-mitochondrial ATP turnover (H+ accumulation) and exercise duration (H+ flux) may prove to be an optimal stimulus. We hypothesised that more intervals performed just above the lactate threshold (LT) would produce greater changes in MCT content than fewer intervals closer to Wpeak. We also investigated if six weeks of training cessation would be sufficient for MCT content to return to baseline.Using a two-group parallel design, 16 men (23 ± 5 y, mean ± SD) completed 4 weeks of work-matched HIIT at either 20% (LO, n=8) or 94% (HI, n=8) of the difference between LT and Wpeak. VO2peak was measured pre- (0wk) and post-training (+4wk). Repeated-sprint ability (RSA) was assessed at 0wk, +4wk, and after 6 weeks training cessation (+10wk). Western blotting was used to measure MCT1 and MCT4 abundance in muscle samples taken at 0wk, +2wk, +4wk, and +10wk. Data were analysed using a two-way mixed design ANOVA. Cohen’s d effect sizes (ES) are presented as (ES; 90% confidence interval) of the difference scores.MCT1 abundance (Fig.1) at +4wk increased more for LO than HI (ES=0.47; -1.35 to 2.28). MCT4 changes followed different time-courses for the two groups (P=0.007). HI showed an improvement in MCT4 at +2wk but had lost this improvement at +4wk, whereas LO improved only at +4wk (Fig.2). MCT abundance returned to baseline for both groups at +10wk. LT improved in response to training more for LO than HI (14 ± 13% vs 2 ± 9%; ES=0.36; -0.03 to 0.75). The modest increases in Wpeak showed no difference between groups (3 ± 4% and 3 ± 6% for LO and HI respectively). There was little change in total work (2 ± 7% and -2 ± 6% for LO and HI respectively) or fatigue decrement during the RSA test. There were no changes in VO2peak for either group.The large but variable increases in MCT1 abundance for both groups provides unclear evidence as to whether prolonged H+ flux is a key factor for upregulation. Some of the equivocation in MCT4 training adaptation may be explained by differing time-courses of adaptation depending on the intensity of HIIT. The reduction in MCT4 at +4wk and lack of improvement in LT compared to LO hint that 4 weeks of HI training may have caused overreaching. Finally, 6 weeks of training cessation resulted in MCT protein abundance returning to baseline for both groups.