To examine the effect of vasopressin on the sarcolemmal ATP-sensitive K (K_ATP) channel, cell-attached, inside-out and open-cell-attached methods of patch clamp techniques were used in isolated guinea pig ventricular myocytes. Suppressing both glycolytic and oxidative ATP production attained K_ATP channel activation. In the cell-attached mode, vasopressin inhibited K_ATP channels in a concentration-dependent manner with an IC₅₀ of 15.1 ±1.8 nmol/L. In the inside-out configuration, vasopressin failed to block K_ATP channels. In the cell-attached mode, manning compound (1 μmol/L), a V₁ receptor-selective antagonist, blocked the inhibitory action of vasopressin, although OPC-31260 (1 μmol/L), a V₂ receptor-selective antagonist could not affect the action of vasopressin. In addition, vasopressin lost its inhibitory action on K_ATP channels when the channel was activated by pinacidil, a K channel opener and in the open-cell-attached mode effected by streptolysin-O. Thus, the inhibitory action of vasopressin K_ATP channels may occur via V₁ receptor related mechanism. (Circ J 2002; 66: 277 - 282)