To investigate the hypothesis that an increase in plasma volume (PV) is obligatory in reducing the cardiovascular drift that is associated with prolonged exercise following training, a plasma expander (Macrodex) was used to acutely elevate PV. Eight untrained volunteers [maximal oxygen consumption; VO2max 45.2 (2.2) ml x kg(-1) x min(-1), mean (SE)] cycled for 2 h [at 46 (4)% VO2max] in ambient conditions either with no PV expansion (CON) or following PV expansions of either 14% (LOW) or 21% (HIGH). During CON, heart rate (HR) increased (P < 0.05) from 147 (2.4) beats x min(-1) to 173 (3.6) beats x min(-1) from 15 to 120 min of exercise. Both LOW and HIGH conditions depressed (P < 0.05) HR, an effect that was manifested following 15 min of exercise. In contrast, stroke volume (SV) was elevated following PV expansion, with values (ml) of 89.6 (6.8), 97.8 (5.9) and 104 (4.6) noted by 15 min of exercise for CON, LOW and HIGH conditions, respectively. Acute PV expansion, regardless of magnitude, also resulted in elevations in cardiac output (Qc). These differences between conditions persisted throughout the exercise, as did the elevation in Qc that was noted with LOW and HIGH conditions. No difference between Qc, HR or SV was found between LOW and HIGH. In addition, neither LOW nor HIGH conditions altered the change in rectal temperature that was observed during exercise. These results demonstrate that, at least for moderate exercise performed in ambient conditions, PV expansion serves only to alter cardiac function (Qc, HR, SV) early in exercise, and not to attenuate the drift that occurs as the exercise is prolonged.