Emerging evidence suggests that nitric oxide (NO) plays a pivotal role in the mechanism of vascular hyporesponsiveness contributing to microgravity-induced orthostatic intolerance. The cellular and enzymatic source of the NO, however, remains controversial. In addition, the time course of the endothelial-dependent contribution remains unstudied. We tested the hypotheses that the change in vasoresponsiveness seen in acute (3-day) hindlimb unweighted (HLU) animals is due to an endothelium-dependent mechanism and that endothelial-dependent attenuation in vasoreactivity is due to endothelial nitric oxide synthase (NOS-3) dependent activation. Vasoreactivity was investigated in rat aortic rings following acute HLU treatment. Dose responsiveness to norepinepherine (NE) was depressed after 3-day HLU [1,338 ± 54 vs. 2,325 ± 58 mg at max (NE), HLU vs. C, P < 0.001]. However, removal of the endothelium restored the vascular contractility to that of C. In addition, 1H-oxadiazole quinoxalin-1-one (ODQ), a soluble guanylyl cyclase inhibitor, restored the reduced vasoconstrictor responses to phenylephrine (PE) seen in 3-day HLU rings (1.30 ± 0.10 vs. 0.53 ± 0.07 g, HLU + ODQ vs. HLU, P = 0.0001). Ca+ dependent nitric oxide synthase (NOS) activity was increased, as was vascular NO products as a result of HLU. While NOS-3 expression was not increased in HLU rats, phosphorylation of NOS-3 at serine-1177 (an activator of NOS-3) was increased while phosphorylation of serine-495 (an inactivator of NOS-3) was decreased. These findings demonstrate that changes in vasoresponsiveness in the acute HLU model of microgravity are due to an upregulation of the endothelial-dependent NO/cGMP pathway through NOS phosphorylation. © 2010 Springer-Verlag.