Peak oxygen uptake (VO2PEAK) in endurance exercise trained (ET =8) subjects (VO2PEAK = 61.7 1.6 ml 02.kg.min-1) was compared to the V02 PEAK of untrained (UT = 8) subjects (V02 PEAK = 38.4 1.7 ml 02 -kg.min1) after four hours of -6* head-down rest (HDR).Although both groups showed a reduction in blood volume (BV) following HDR, this decrement was greater for ET subjects (delta BV = -3.23 0.46 mi/kg; P <0.05). The ET subjects had a greater decrease in VO2=(delta 02E -5.58 1.05 ml 02-kg.min-1; P <0.05) than their UT peers (VO2PEAK = -2.44 0.79 ml02-kg.min-1). These data suggest that the greater reductions in VO2PE, observed for the ET group were associated with a greater BV loss resulting from 4 h of HDR prior to exercise.