The aim of present study was to explore the vasodilatation mechanism of angiotensin II (AngII) at the molecular level by investigating the effect of AngII on large-conductance Ca²⁺-activated potassium channels (BK(Ca)) in human mesenteric artery smooth muscle cells. The effect of AngII on BK(Ca) was observed by using patch clamp single channel recording technique and amphotericin-perforated whole-cell recording technique. AngII type 1 receptor (AT₁R) and AngII type 2 receptor (AT₂R) mRNA expression in human mesenteric artery was detected by RT-PCR. In cell-attached patch (Vm = +40 mV), AngII (100 nmol/L) had no significant effect on BK(Ca). After pretreatment with Valsartan (a specific inhibitor of AT₁R, 10 μmol/L), 25, 100 and 250 nmol/L AngII stimulated BK(Ca) activity significantly in a dose response manner. After pretreatment of Valsartan, AngII (100 nmol/L) enhanced BK(Ca) open probability (NP(O)) from 0.010 ± 0.003 to 0.039 ± 0.015, decreased the mean close time (T(C)) of BK(Ca) markedly from (2 729.5 ± 808.6) ms to (487.7 ± 182.5) ms (n = 11, P < 0.05) , but AngII had no significant influences on the amplitude (Amp) and the mean open time (T(O)) of BK(Ca). Further PD123,319 (a specific inhibitor of AT₂R) treatment prevented the stimulatory effect of AngII: PD123,319 decreased the NP(O) of BK(Ca) from 0.016 ± 0.003 to 0.004 ± 0.001 (n = 5, P < 0.05), but had no significant influences on Amp, T(O) and T(C) of BK(Ca). In addition, after pretreatment with Valsartan and PD123,319, AngII (100 nmol/L) had no significant effect on BK(Ca). In the amphotericin-perforated whole-cell patch-clamp configuration, after pretreatment with Valsartan, the current density of BK(Ca) at the voltage of -60 - +30 mV had no significant changes before and after adding 100 nmol/L AngII, but the current density of BK(Ca) at the voltage of +40 mV, +50 mV and +60 mV increased significantly after adding 100 nmol/L AngII, from (9.03 ± 2.23) pA/pF, (12.88 ± 2.55) pA/pF and (17.26 ± 2.84) pA/pF to (12.47 ± 2.22) pA/pF, (18.71 ± 2.51) pA/pF and (27.21 ± 3.12) pA/pF (n = 6, P < 0.05), respectively. Using RT-PCR, the AT₁R mRNA and AT₂R mRNA from isolated human mesenteric artery were detected. So we can draw a conclusion, AngII can stimulate BK(Ca) activity in human mesenteric artery smooth muscle cells after pretreatment with Valsartan, which is possibly mediated by AT₂R.