Cardiovascular deconditioning after a 4-hour head down bed rest (HDBR) might be a consequence of the time of day relative to pre-HDBR testing, or simply 4-hours of confinement and inactivity rather than the posture change. Ten men and 11 women were studied during lower body negative pressure (LBNP) before and after 4-hours HDBR and 4-hours seated posture (SEAT) as a control for time of day and physical inactivity effects to test the hypotheses that cardiovascular deconditioning was a consequence of the HDBR posture, and that women would have a greater deconditioning response. Following HDBR, men and women had lower blood volume, higher heart rate with a greater increase during LBNP, a greater decrease of stroke volume during LBNP, lower central venous pressure, smaller inferior vena cava diameter, higher portal vein resistance index with a greater increase during LBNP, but lower forearm vascular resistance, lower norepinephrine and lower renin. Women had lower vasopressin and men had higher vasopressin after HDBR, and women having lower pelvic impedance and men higher pelvic impedance. Following SEAT, brachial vascular resistance was reduced, thoracic impedance was elevated, the reduction of central venous pressure during LBNP was changed, women had higher angiotensin II whereas men had lower levels, and pelvic impedance increased in women and decreased in men. Cardiovascular deconditioning was greater after 4-hours HDBR than after SEAT. Women and men had similar responses for most cardiovascular variables in the current study that tested the responses to LBNP after short-duration HDBR compared to a control condition.