BACKGROUND: Selective 2-agonists cause decrease in serum K+ concentration. Midazolam is an anxiolytic, sedative, and amnestic drug. Premedication of midazolam prevents increase of catecholamine with anxiety. Clonidine, alpha 2-adrenergic receptor agonist, supresses sympathetic outflow from central nervous system. So we can expect that premedication of clonidine or midazolam will prevent hypokalemia before induction of anesthesia. METHODS: Twenty two patients received 300 mcg clonidine per oral, 22 patients 0.05 mg/kg midazolan IM and 22 patients had no premedication. We measured serum K+ level at out-patient Department (T1), at 11:00 P.M. of the day before surgery (T2) and immediately before induction of anesthesia (T3). RESULTS: Serum K+ levels at T2 decreased compared to serum K+ level at T1 in all groups. Serum K+ levels T3 decreased compared to serum K+ level at T2 in control and midazolam groups but clonidine group did not decrease in serum K+ level. CONCLUSIONS: We can not prevent decrease of serum K+ level with premedication of midazolam but we can prevent decrease of serum K+ level with premedication of clonidine. So premedication of clonidine is more effective than midazolam in prevention of hypokalemia before induction of anesthesia.