Studies of the cardiovascular response to lower body negative pressure (LBNP) in men and women have suggested that women may have less tolerance to LBNP than men, although tolerance per se was not determined. To investigate the effect of gender on tolerance to LBNP, 10 men 10 women were subjected to increasing levels of LBNP until presyncopal symptoms developed. The cumulative stress index (CSI) score was determined, as were cardiovascular variables. Women had 62% less tolerance to LBNP with a CSI of 412 +/- 43 mmHg/min compared with a CSI of 1,070 +/- 149 mmHg/min for men. Cardiovascular changes associated with LBNP were similar for men and women when expressed relative to the occurrence of presyncope, but women had a higher heart rate response when the data were expressed at absolute levels of LBNP (-30 and -50 mmHg LBNP). Thus men and women had similar cardiovascular adjustments to the LBNP, with the changes in women occurring lower levels of LBNP. These data are important in a consideration of the development of antigravitational countermeasures for women. These data raise questions as to the manner in which blood pools within the lower body in men and women under LBNP.