Idazoxan, $alpha$$_2$-adrenergic antagonist, produced antidiuretic action by administration into the vein and diuretic action only in ipsilateral kidney by injection into a renal artery in dog. These studies were performed for investigation of mechanism on the renal action induced by idazoxan. Antiduretic action by idazoxan given into vein and diuretic action only in ipsilateral kidney by idazoxan injected into a renal artery were blocked entirely by renal denervation. Antidiuretic action of idazoxan given into the vein was weakened by UK 14,304, $alpha$$_2$-adrenergic agonist, pretreated into the vein. Above results suggest that antidiuretic action of idazoxan given into the vein is caused by blocking of $alpha$$_2$-adrenergic receptor, diuretic action only in ipsilateral kidney of idazoxan injected into a renal artery by blocking of $alpha$$_2$-adrenergic receptor in the kidney.