Aim: It is well known that oral ingestions of bicarbonate (HCO3-) lead to a higher efflux of lactate and H+-ions out of the skeletal muscle. Less is known about the effects of HCO3- ingestion on the lactate-H+-co-transport in red blood cells (RBC) via MCT1. Therefore the purpose of this study was to investigate the effects of an oral HCO3- ingestion on lactate-H+-co-transport in RBC during exercise. Methods: 11 male athletes performed two separated sessions each consisting of four 30 sec lasting bike sprints on a cycle ergometer each interrupted by 5 min rest. Over a period of 90 min before the intervention started subjects ingested either 0.3 g/kg of NaHCO3 (Bic) or 2 g of a Placebo (P) (CaCO3). Arterialized capillary blood samples were taken from both hyperemized earlobes to measure lactate and blood gases (HCO3-, H+, pH) in whole blood, plasma and RBC at several points of time. Results: At rest H+-ion concentration ([H+]) in RBC (71.7 ± 3.5 nmol/L) was nearly twice as high as in plasma (40.8 ± 1.8 nmol/L). After ingestion of bicarbonate the [H+] in RBC (35.3 ± 1.0 nmol/L) and in plasma (65.7 ± 3.5 nmol/L) decreased but no significant shift in the ratio occurred. After the first sprint the [H+] in both compartments increased significantly in both interventions, with no change in ratio again. The ratio of H+-ions nearly stayed the same during the whole test. At rest the [La] in plasma was twice as high as that of RBC [La] in both interventions (ratio: 0.46). After the first sprint the ratio decreased significantly in both interventions (Bic: 0.35 ± 0.05 vs. P: 0.34 ± 0.03). Conclusion: The main finding of our study was that an oral ingestion of NaHCO3 during exercise has no effects on lactate-H+-co-transport in RBC. After the ingestion a new donnan-equilibrium between plasma and RBC was adjusted. The shift in the lactate-ratio was not mirrored in the H+-ratio. The H+-ratio is not delayed because the acid enters the RBC as CO2 released by the HCO3- in the plasma.