With this study we tested the hypothesis that six weeks of endurance training increases maximal cardiac output (Q_max) relatively more by elevating blood volume (BV) than by inducing structural and functional changes within the heart. Nine healthy but untrained volunteers (VO₂max 47 ± 5 ml.min^-1.kg^-1) underwent supervised training (60 min; 4 times weekly at 65% VO₂max for six weeks) and Q_max was determined by inert gas re-breathing during cycle ergometer exercise before and after the training period. After the training period, blood volume (determined in duplicates by CO re-breathing) was re-established to pre-training values by phlebotomy and Q_max was quantified again. Resting echography revealed no structural heart adaptations as a consequence of the training intervention. Following the training period, plasma volume (PV), red blood cell volume (RBCV) and BV increased (p<0.05) by 147 ± 168 (5 ± 5 %), 235 ± 64 (10 ± 3 %) and 382 ± 204 ml (7 ± 4 %), respectively. VO₂max was augmented (p<0.05) by 10 ± 7 % following the training period and decreased (p<0.05) by 8 ± 7 % with phlebotomy. Concomitantly, Q_max was increased (p<0.05) from 18.9 ± 2.1 to 20.4 ± 2.3 l.min^-1 (9 ± 6 %) as a consequence of the training intervention, and following normalization of BV by phlebotomy Q_max returned to pre training values (18.1 ± 2.5 l.min^-1; 12 ± 5 % reversal). Thus, the exercise training induced increase in BV is the main mechanism increasing Q_max following six weeks of endurance training in previously untrained subjects.