This study tested the hypothesis that cardiovascular and hormonal responses to lower body negative pressure (LBNP) would be altered by 4-h head down bed rest (HDBR) in 11 healthy young men. In post-HDBR testing, 3 subjects failed to finish the protocol due to pre-syncopal symptoms, heart rate was increased during LBNP compared to pre-HDBR, mean arterial blood pressure was elevated at 0, -10 and -20 mmHg and reduced at -40 mmHg, central venous pressure (CVP) and cardiac stroke volume were reduced at all levels of LBNP. Plasma concentrations of renin, angiotensin II and aldosterone were significantly lower after HDBR. Renin and angiotensin II increased in response to LBNP only post-HDBR. There was no effect of HDBR or LBNP on norepinephrine while epinephrine tended to increase at -40 mmHg post-HDBR (p=0.07). Total blood volume was not significantly reduced. Splanchnic blood flow taken from ultrasound measurement of the portal vein was higher at each level of LBNP post- compared to pre-HDBR. The gain of the cardiopulmonary baroreflex relating changes in total peripheral resistance to CVP was increased after HDBR, but splanchnic vascular resistance was actually reduced. These results are consistent with our hypothesis and suggest that cardiovascular instability following only 4-hours HDBR might be related to altered hormonal and/or neural control of regional vascular resistance. Impaired ability to distribute blood away from the splanchnic region was associated with reduced stroke volume, elevated heart rate and the inability to protect mean arterial pressure.