High (n = 7, 25 +/- 2 yr) and low (n = 8, 26 +/- 3 yr) lower body negative pressure (LBNP) tolerant men were exposed to -15 mmHg (for 12 min) followed by -50 mmHg (for 21 min) to test the hypothesis that heart rate (HR) and blood pressure (BP) data from acute exposure to LBNP would not discriminate between the higher and lower tolerance men. Central venous pressure (CVP), HR, and systolic (SBP) and diastolic (DBP) blood pressures measured before and at 15-s intervals during LBNP and calculated mean arterial pressure (MAP), pulse pressure (PP), and work of the heart (HW) were analyzed using ANOVA (p < or = 0.05). There were no significant changes in HR, SBP, DBP, MAP, PP, or HW during exposure to -15 mmHg LBNP. Throughout -50 mmHg LBNP, there were no significant changes in SBP, MAP, PP, or HW, but HR increased significantly (high tolerance by 30%, low tolerance by 40%) with no difference between groups. Diastolic blood pressure changed by +7.6 % (NS) in the high group and by -3.3% (NS) in the low group; the initial exposure to -50 mmHg resulted in a significant difference between groups for the first 45 s. Central venous pressure decreased significantly at -15 mmHg (high group by -33%, low group by -38 %) and at -50 mmHg (high group by -70%, low group by -73%) with no difference between groups. Thus, HR and BP responses at -15 and -50 mmHg of LBNP for 30 min do not discriminate between the high and low tolerant men and questions the validity and usefulness of the clinical stand test to predict orthostatic tolerance.