Microgravity is associated with an impaired stroke volume and, therefore, cardiac output response to orthostatic stress. We hypothesized that a decreased venous filling pressure due to increased venous compliance may be an important contributing factor in this response. We used a constant flow, constant right atrial pressure cardiopulmonary bypass procedure to measure total systemic vascular compliance (C(T)), arterial compliance (C(A)), and venous compliance (C(V)) in seven control and seven 21-day hindlimb unweighted (HLU) rats. These compliance values were calculated under baseline conditions and during an infusion of 0.2 microg*kg(-1)*min(-1) norepinephrine (NE). The change in reservoir volume, which reflects changes in unstressed vascular volume (DeltaV(0)) that occurred upon infusion of NE, was also measured. C(T) and C(V) were larger in HLU rats both at baseline and during the NE infusion (P < 0.05). Infusion of NE decreased C(T) and C(V) by ~20% in both HLU and control rats (P < 0.01). C(A) was also significantly decreased in both groups of rats by NE (P < 0.01), but values of C(A) were similar between HLU and control rats both at baseline and during the NE infusion. Additionally, the NE-induced DeltaV(0) was attenuated by 53% in HLU rats compared with control rats (P < 0.05). The larger C(V) and attenuated DeltaV(0) in HLU rats could contribute to a decreased filling pressure during orthostasis and thus may partially underlie the mechanism leading to the exaggerated fall in stroke volume and cardiac output seen in astronauts during an orthostatic stress after exposure to microgravity.