In order to identify functional sulphonylurea receptor (SUR.x) subunits of native ATP-sensitive K+ channels (KATP channels) in mouse portal vein, the effects of ZD0947, a SUR.x modulator, were investigated on spontaneous portal vein contractions, macroscopic membrane currents and unitary currents recorded (using patch-clamp techniques) in freshly dispersed mouse portal vein myocytes. Spontaneous contractions in mouse portal vein were reversibly reduced by ZD0947 in a concentration-dependent manner (Ki =293 nM). The relaxation elicited by 3 μM ZD0947 was antagonized by the additional application of glibenclamide (300 nM), but not gliclazide (100–300 nM). In the conventional whole-cell configuration, 100 μM ZD0947 elicited inward glibenclamide-sensitive currents at a holding potential of −60 mV that demonstrated selectivity for K+(i.e. KATP currents). The peak amplitude of the membrane current elicited by 30 μM or 100 μM ZD0947 was smaller than that elicited by 100 μM pinacidil at −60 mV. In the cell-attached mode, 100 μM ZD0947 activated glibenclamide-sensitive K+ channels with a conductance (35 pS) similar to that of recombinant Kir6.1/SUR2B channels that were expressed in HEK293 cells and activated by 100 μM ZD0947. These results demonstrate that ZD0947 caused a significant vascular relaxation through the activation of KATP channels and that SUR2B may be the major functional subunit of SUR.x in mouse portal vein KATP channels, based on its pharmacological selectivity.