Regulation of cerebral blood flow is of great importance for normal brain function. Neurovascular coupling is responsible for matching blood flow and neuronal activity. It has been suggested that release of K+ from astrocytic endfeet, mediating neuronal signaling, is sensed by the adjacent arterioles and is a component of neurovascular signaling. Increased interstitial [K+] hyperpolarizes smooth muscle cells (SMC), dilate arterioles and increase blood flow to meet the metabolic demand of active neurons. Familial hemiplegic migraine type 2 (FHM2) is a classical migraine with aura. FHM2 is associated with single-point mutations in the Na+,K+-ATPase α2 isoform gene including G301R mutation. Dilation of the middle cerebral artery (MCA) in the affected hemisphere was shown in FHM2 patients during a migraine attack. We hypothesize that this is due to abnormal K+ signaling between astrocytes and SMC leading to changes in neurovascular coupling. A mouse model bearing the G301R mutation phenocopying several FHM2-relevant disease traits was used in this study. MCA from both wild type and FHM2 mice were compared in-vitro in isometric myographs. Pre-constricted MCA were relaxed by increasing bath concentrations of K+, [K+]out. This protocol was repeated with 10 μM ouabain, a Na+,K+-ATPase inhibitor, 30 μM BaCl2, a K+- inward rectifying channel (Kir) inhibitor and after denudation of endothelium. In some experiments, membrane potential was measured simultaneously with isometric force recordings. Expression of Kir was semi-quantified immunohistochemically in whole-mount preparations and in Western Blots (WB). Arteries from FHM2 mice had stronger K+-induced relaxation in comparison with WT at [K+]out in the range of 7 – 18 mM. There was no difference in the relaxation between the sexes of same genotype. Ouabain had no effect on the K+-induced relaxation of FHM2 arteries and only slightly inhibited the relaxation of WT. BaCl2 suppressed the K+-induced relaxation almost completely in both groups. Accordingly, FHM2 arteries hyperpolarize stronger at 9 mM [K+]out than WT, and this difference was abolished by BaCl2. This was not due to changes in the Kir expression in SMC, as is evident from whole-mount staining and WB analyses. Removal of the endothelium suppressed the relaxation primarily in the FHM2 group and abolished the difference between the two groups. FHM2-associated mutation in the Na+,K+-ATPase α2 isoform leads to abnormal sensitivity of MCA to interstitial K+ possibly due to increased contribution of endothelial Kir. This might be the mechanism behind hyperperfusion associated with the headache stage of migraine attacks.