The aim was to evaluate the cardiodynamic adjustment during 4 days of prolonged exercises and to check if the plasma volume (PV) expansion which is observed generally during such events plays a role in this adaptation. Thirteen subjects exercised 5 hours per day on a cycle ergometer alternately with a treadmill for 4 days (D1 to D4) (6 x 50 min sessions per day). The individual cycle ergometer load and the treadmill speed were unchanged during each exercise session and throughout all the sessions, and corresponded to a moderate exercise intensity: 58 - 63 % of peak oxygen uptake (VO (2)peak). Heart rate (HR) was recorded every 15 s during each exercise session and VO (2) was measured from the expired air at the beginning and the end of each exercise session. Relative PV changes were measured from haematocrit and haemoglobin changes in the morning before the exercise bouts. No significant changes of VO (2) were observed between the first and the last exercise session i. e. for cycling: 2.1 +/- 0.2 l/min and for running: 2.4 +/- 0.3 l/min. Between the first and the last day, HR decreased from 143 to 129 bpm for cycle (p < 0.0001) and from 147 to 137 bpm (p = 0.01) for treadmill. As compared to D1, PV increased gradually from D2 (+ 1.8 % +/- 4.7 %) to D4 (+ 8.5 % +/- 4.7 %). The individual PV increases were significantly correlated with cycling HR decreases from D1 to D4 (r (2) = 0.40, p = 0.02). In conclusion, the 4 days' prolonged exercise induced a HR decrease during submaximal exercise without VO (2) drift. Here we suggested that this HR decline could be in part linked to the transient PV expansion.