The response of the regional cerebral blood flow (rCBF) to brain topical superfusion of 20 mM K⁺ was characterized in a closed cranial window preparation in barbiturate anesthetized and ventilated rats; Increasing K⁺ in the artificial cerebrospinal fluid (ACSF) induced a rCBF elevation (measured by laser–Doppler flowmetry) of +85 ± 37% above baseline (n = 19). This elevation was stable for >3 h with continuous superfusion of increased K⁺ (n = 5) and partially reversible to a level of + 18 ± 19% above baseline when returning to a physiological K⁺ concentration. Nitric oxide synthase (NOS) inhibition by brain topical superfusion with Nω-nitro-L-arginine (L-NA) revealed (a) Addition of L-NA to high-potassium ACSF reduced the rCBF increase from + 94 ± 36% to + 21 ± 18% (p ≤ 0.01, n = 7). (b) When L-NA was superfused for 60 min before increasing K⁺, rCBF decreased to – 17 ± 7% below baseline. Subsequent coapplication of L-NA and increased K⁺ induced only an elevation of +7 ± 4% above baseline (n = 4). (c) When the NO donor S-nitroso-N-acetylpenicillamine (SNAP) was added during NOS inhibition to restore basal tissue NO levels, the resultant level of rCBF was +28 ± 54% above baseline. Subsequent increase of K⁺ in the presence of NOS inhibition and SNAP elevated rCBF to + 137 ± 89% above baseline (n = 4). Statistical analysis comparing K⁺-induced elevation of rCBF (a) without any added drugs, (b) in the presence of NOS inhibition with L-NA, and (c) in the presence of both NOS inhibition and SNAP revealed that K⁺-induced elevation in the presence of NOS inhibition was significantly reduced (p ≤ 0.05) whereas no statistical difference was found between K⁺-induced elevation of rCBF without drugs compared with the K⁺-induced elevation of rCBF in the presence of L-NA and SNAP. We conclude that NO is a modulator of the rCBF elevation to increased extracellular K⁺ concentration.