Objective: To examine the impact of a 6-week endurance training on red blood cell (RBC) aging and deformability of healthy participants to detect possible improved hemorheological and performance-related adaptations.Methods: A total of 31 participants (17 females and 14 males) performed a 6-week moderate training protocol (three 1-h running sessions per week at 70％ of maximal heart rate).Blood was sampled before and after the training.RBCs from each participant were fractioned according to density and age into 4 RBC subfractions.Subfractions were examined for changes of RBC properties,including aging distribution,RBC deformability,RBC microparticles,and phosphatidylserine concentrations.RBC and plasma nitrite levels were measured as indicators of nitric oxide metabolism.Results: Aerobic performance,peak oxygen consumption,ventilatory thresholds,velocity at the aerobic-anaerobic threshold,and lactate at exhaustion improved after training.The relative amount of both young RBCs and old RBCs increased,and the amount of the main RBC fraction decreased.Phosphatidylserine externalization and RBC-derived microparticles decreased.Overall deformability expressed as shear stress required to achieve half-maximum deformation to theoretical maximal elongation index at infinite shear stress improved in unfractioned RBCs (p ＜ 0.001).Nitrite decreased in total (p=0.001),young (p ＜ 0.001),main (p ＜ 0.001),and old (p=0.020) aged RBCs and in plasma (p =0.002),but not in very old RBCs.Conclusion: These results indicate that non-endurance-trained healthy participants benefit from a regular moderate running training program because performance-related parameters improve and a younger RBC population with improved RBC properties is induced,which might support oxygen supply in the microcirculation.