The addition of epinephrine to local anesthetics for peipheral nerve blocks reduces the risk of local anesthetic toxicity by delaying systemic absorption and prolongs block duration. But there is general agreement that epinephrine causes a decrease in plama K+ in humans that can be associated with a variety of cardiac dysrhythmias. Clonidine, a selective 2 adrenergic agonist, has been reported to increase the local anesthetic effect. And the addition of clonidine to local anesthetics in peripheral nerve block prolongs block duration and postoperative analgesia. The admixture of clonidine or epinephrine to bupivacaine for brachial plexus block was studied with regard to plasma potassium concentrations and hemodynsmie changes. Thirty-three patients of ASA physical statue I and II received an admixture of clonidine (150g; n=11), epinephrine (200g; n=11), or normal saline (placebo; n=11) to 30 ml of 0.5% bupivacaine in a randomized, double blind fashion. There were no differences in arterial blood pressure and heart rate among the three groups. In patients who had received epinephrine admixture, decrease of plasma potassium at 15min after block was significant compared with patients who had received clonidine. Clonidine may be a useful adjunt to loeal anesthetics in those patients in whom the administration of epinephrine is contraindicated.