The cardiovascular function is one of the main functions disturbed by microgravity. It is particularly affected by the astronaut's return to Earth, where one of the symptoms of the cardiovascular adaptation syndrome is orthostatic hypotension; the clinical consequence can be presyncopal state or a syncope. Lower body negative pressure (LBNP) is intended to stimulate the venous system of the lower limbs. Studies performed in the U.S. have shown that LBNP constitutes an efficient countermeasure, but this approach is impractical because 4 to 6 h/d of application are required. Five volunteers took part in two recent antiorthostatic bed rest experiments for 30 days. In the first experiment, three subjects were submitted to several sessions of LBNP per day and two others were controls; in the second, the LBNP group of the first experiment became control and vice versa. Two orthostatic investigations were performed: 5 d before bed rest; and at the end of the 30-d bed rest period. The results showed that: 1) when the subjects were controls, a high orthostatic hypotension post bed rest with three syncopes and one presyncopal state during the first minutes of the tilt test appeared; 2) when the subjects were submitted to LBNP sessions, no orthostatic hypotension was noted. These two experiments proved the beneficial effects of the LBNP as a countermeasure against orthostatic hypotension.