The effects of clonidine as a premedicant were studied during isoflurane-indueed hypotension(IIH). Twenty-four adult patient, ASA grade Ⅰ-Ⅱ, scheduled for elective cerebral surgery, were randomly allocated into two groups. Atropine 0.5mg and luminal sodium 100mg were given I. M. as premedication in group Ⅰ 30 mins before induction, and clonidine 5?g?kg~(-1)P. O. was combined in group Ⅱ 90 mins prior to induction. The methods of both groups were similar in induction and maintenance of anesthesia. The status of hypotension was induced and kept by inhaltion of isoflurane, with MAP being decreased by 25% or so. The arterial plasma Concentrations of adrenaline and noradrenaline were measured by electrochemistry method, at preoperation, prior to hypotension, 15 and 30 mins following IIH and 30 mins after MAP recovery, respectively. The results revealed that as compared with the values in group Ⅰ, the inspired and end-tidal concentrations of isoflurane reduced significantly during induction and maintenance of controlled hypotension (P0.05). It is suggested that clonidine premedication can increase the efficiency of isoflurane for controlled hypotension and do not affect the plasma catecholamine concentration and airway function.