Adequate hydration is essential for normal brain function and dehydration induces cognitive deterioration. In addition, dehydration has emerged as a stroke risk factor. However, it is unknown whether alterations in cerebrovascular regulation are responsible for these effects. To address this issue, C57Bl/6 mice were water deprived for 24 or 48 hours and somatosensory cortex blood flow was assessed by laser-Doppler flowmetry in a cranial window. Dehydration increased plasma osmolality and vasopressin levels, and suppressed the increase in blood flow induced by neural activity, by the endothelium-dependent vasodilator acetylcholine and the smooth muscle relaxant adenosine. The cerebrovascular dysfunction was associated with oxidative stress and cognitive deficits, assessed using the Y maze. The vasopressin 1a receptor antagonist SR49059 improved the dehydration-induced oxidative stress and vasomotor dysfunction. Dehydration upregulated endothelin-1 in cerebral blood vessels, an effect blocked by SR49059. Furthermore, the endothelin A receptor antagonist BQ123 ameliorated cerebrovascular function. These findings show for the first time that dehydration alters critical mechanisms regulating the cerebral circulation through vasopressin and oxidative stress. The ensuing cerebrovascular dysregulation may alter cognitive function and increase the brain's susceptibility to cerebral ischemia.Journal of Cerebral Blood Flow & Metabolism advance online publication, 12 February 2014; doi:10.1038/jcbfm.2014.24.