The purpose of this study is to clarify which K + channels contribute to the acetylcholine (ACh)-induced vasodilation from the diameter changes in arterioles of the guinea-pig choroid.The choroid was isolated from the guinea-pig eyeball, pinned flat on a silicone rubber plate and superfused with warmed oxygenated (35°C) Krebs solution. Diameters of choroidal arterioles were measured using video microscopy and a computer program for analysis. The effects of K + channel inhibitors (glibenclamide, tetraethylammonium [TEA], apamin and charybdotoxin [ChTX]) on the ACh-induced vasodilation were examined in arterioles which had been constricted by either norepinephrine (NE) or high K + solution.In NE (10 −5 m)-constricted arterioles, the combination of nitroarginine (10 −4 m) and indomethacin (10 −5 m) reduced ACh (10 −6 m)-induced vasodilatation by 24%. When high K + solution was used to constrict the arterioles, ACh-induced vasodilation was abolished by nitroarginine and indomethacin. In the presence of nitroarginine and indomethacin, the ACh-induced dilatation of NE-constricted arterioles was attenuated by TEA (10 −3 m), apamin (10 −7 m), and ChTX (10 −7 m) but not by glibenclamide (2×10 −5 m). Simultaneous application of apamin and ChTX inhibited the ACh (10 −6 m)-induced dilatation by 85%.In arterioles of guinea pig-choroid, nitric oxide and prostacyclin are not main mediators in ACh-induced vasodilation. Simultaneous activation of a set of Ca 2+ -sensitive K + channels may take most part of ACh-induced vasodilation.