This study assessed the respective contributions of haematological and skeletal muscle adaptations to any observed improvement in peak oxygen uptake (V_O2peak) induced by endurance training (ET). V_O2peak , peak cardiac output (Q_peak), blood volumes and skeletal muscle biopsies were assessed prior (pre) to and after (post) 6 weeks of ET. Following the post-ET assessment, red blood cell volume (RBCV) reverted to the pre-ET level following phlebotomy and V_O2peak and Q_peak were determined again. We speculated that the contribution of skeletal muscle adaptations to an ET-induced increase in V_O2peak could be identified when offsetting the ET-induced increase in RBCV. V_O2peak, Q_peak , blood volumes, skeletal muscle mitochondrial volume density and capillarization were increased after ET. Following RBCV normalization, V_O2peak and Q_peak reverted to pre-ET levels. These results demonstrate the predominant contribution of haematological adaptations to any increase in V_O2peak induced by ET. It remains unclear whether improvements in peak oxygen uptake (V_O2peak) following endurance training (ET) are primarily determined by central and/or peripheral adaptations. Herein, we tested the hypothesis that the improvement in V_O2peak following 6 weeks of ET is mainly determined by haematological rather than skeletal muscle adaptations. Sixteen untrained healthy male volunteers (age = 25 ± 4 years, V_O2peak = 3.5 ± 0.5 l min^-1) underwent supervised ET (6 weeks, 3-4 sessions per week). V_O2peak, peak cardiac output (Q_peak), haemoglobin mass (Hb_mass) and blood volumes were assessed prior to and following ET. Skeletal muscle biopsies were analysed for mitochondrial volume density (Mito_VD), capillarity, fibre types and respiratory capacity (OXPHOS). After the post-ET assessment, red blood cell volume (RBCV) was re-established at the pre-ET level by phlebotomy and V_O2peak and Q_peak were measured again. We speculated that the contribution of skeletal muscle adaptations to the ET-induced increase in V_O2peak would be revealed when controlling for haematological adaptations. V_O2peak and Q_peak were increased (P < 0.05) following ET (9 ± 8 and 7 ± 6%, respectively) and decreased (P < 0.05) after phlebotomy (-7 ± 7 and -10 ± 7%). RBCV, plasma volume and Hb_mass all increased (P < 0.05) after ET (8 ± 4, 4 ± 6 and 6 ± 5%). As for skeletal muscle adaptations, capillary-to-fibre ratio and total Mito_VD increased (P < 0.05) following ET (18 ± 16 and 43 ± 30%), but OXPHOS remained unaltered. Through stepwise multiple regression analysis, Q_peak, RBCV and Hb_mass were found to be independent predictors of V_O2peak. In conclusion, the improvement in V_O2peak following 6 weeks of ET is primarily attributed to increases in Q_peak and oxygen-carrying capacity of blood in untrained healthy young subjects.