Fourteen young male volunteers (mean age 28.1 yr) underwent maximal exercise performance testing and lower body negative pressure (LBNP) challenge to -50 Torr. Two distinct groups, fit (F, n = 8), mean maximal aerobic capacity (VO2max) = 70.2 +/- 2.6 (SE) ml O2 kg-1 X min-1, and average fit (AF, n = 6), mean VO2 max V 41.3 +/- 2.9 ml O2 kg-1 X min-1, P less than 0.001, were evaluated. Rebreathing CO2 cardiac outputs, heart rate (HR), blood pressure (BP), and leg circumference changes were monitored at each stage of progressive increases in LBNP to -50 Torr. The overall hemodynamic responses of both groups of subjects to LBNP were qualitatively similar to previous findings. There were no differences between F and AF in peripheral venous pooling as shown by a leg compliance (delta leg volume/delta LBNP) for the F of 12.6 +/- 1.1 and for the AF 11.6 +/- 2.0, P greater than 0.05. The F subjects had significantly less tachycardic response [delta HR/delta systolic BP of F = 0.7 beats/Torr] to LBNP to -50 Torr than the AF subjects [delta HR/delta systolic BP of unfit (UF) = 1.36 beats/Torr], P less than 0.05. In addition, overall calculated peripheral vascular resistance was significantly higher in the AF subjects (P less than 0.001), and there was a more marked decrease in systolic BP of the F subjects between the LBN pressures of -32 to -50 Torr. We concluded that the reflex response to central hypovolemia was altered by endurance exercise training.