Direct observation was made by using the patch-clamp technique with a specially designed microperfusion system to investigate the effect of acetylcholine (Ach 10~(-6) mol/L) elicited endothelium-derived relaxing factor (EDRF) on the calcium-activated potassium channel (IK(Ca))in the smooth muscle cells of mesenteric resistance vessels in Wistar rats. Activation of IK(Ca) was firstly observed by inducing the elicited EDRF or sodium nitroprusside (SNP 10~(-8) mol/L) under various clamping voltages in cell-attached configuration. While the pipette solution contained KCl 126 mmol/L and the bath solution contained KCl 5.9 mmol/L, two types of conductances of calcium-activated potassium current being 76.4±2.3 pS(mean±S.E. n = 7) and 160.3±7.5 pS (mean±S.E. n= 7) were recorded during the EDRF activation, one type of conductance being 100.5±2.8 pS (mean±S.E. n = 6) was activated by nitric oxide (NO) which is an effective component from SNP. Differences in kinetic characteristics of these channels between Direct observation was made by using the patch-clamp technique with a specially designed microperfusion system to investigate the effect of acetylcholine (Ach 10 -6 mol / L) elicited endothelium-derived relaxing factor (EDRF) on the calcium-activated potassium (IK (Ca)) in the smooth muscle cells of mesenteric resistance vessels in Wistar rats. Activation of IK (Ca) was detected by inducing the elicited EDRF or sodium nitroprusside (SNP 10 ~ (-8) mol / L) While the pipette solution contained KCl 126 mmol / L and the bath solution contained KCl 5.9 mmol / L, two types of conductances of calcium-activated potassium current being 76.4 ± 2.3 pS (mean ± SE n = 7) and 160.3 ± 7.5 pS (mean ± SE n = 7) were recorded during the EDRF activation, one type of conductance being 100.5 ± 2.8 pS (mean ± SE n = 6) was activated by nitric oxide an effective component from SNP. Differences in kinetic characteristics of these channels between