Clonidine, an α2-agonist, increases urine output and electrolyte excretion, and reduces the dose requirement of vasodilator for induced hypotension. We examined whether oral clonidine would provide these effects during hypotensive anesthesia. Twenty patients randomly received oral clonidine either 5μg/kg(clonidine group, n＝10), or none(control group, n＝10). General anesthesia was maintained with isoflurane and 67% N2O. During hypotensive period, mean arterial pressure(MAP) was kept at 50-60 mmHg. Urine output, urine osmolality, urinary excretion of electrolytes, and plasma ADH level were examined. Urine output and absolute urinary excretion of potassium were greater during the first 30 minutes of hypotensive period in the clonidine group than the control group. Five patients in the control group and one in the clonidine group had no urine output during the hypotensive period(p＜0.05). Absolute urinary excretion of sodium and potassium increased in the control group after MAP recovered. However, plasma ADH levels did not change over time in both groups, and were similar between groups. No difference was found between the two groups in the dose requirement of nicardipine during hypotensive period. Oral preanesthetic medication of clonidine 5μg/kg provided a diuretic effect during hypotensive anesthesia.