【Abstract】 Objective To evaluate the influence of dexmedetomidine (highly selectiveａ２-adrenergic receptor agonist) on haemodynamics and bispectral index during anesthesia induction in patients undergoing valve replacement surgery. Methods Tirty NYHA class Ⅱ or Ⅲ patients, aged between 30yr and 70yr, undergoing cardiac value replacement with CPB, were randomly divided into 2 groups (n=15 each): dexmedetomidine group (group D) and control group (group C). In Dex group,pump diluted with saline to 50ml of Dex(the concentration 4ml/ml) 0.5ug/kg was administered in travenously l0 minutes before the induction of anesthesia, while in C group,same volume of 0.9% normal saline was given as control. The patients were premedicated with intramuscular morphine and scopolamine. Anesthesia was induced with iv injection of midazolam 0.06mg/kg, fentanyl 6μg/kg, etomidate 0.3mg/kg and pipecuronium 0.10mg/kg. Anesthesia was maintained with intermittent iv boluses of midazolam, fentanyl, pipecuronium. In two group , heart rate(HR), blood pressure(BP) [systolic blood pressure(SBP), diastolic blood pressure (DBP), mean arterial pressure(MAP)] and bispectral index(BIS) were recorded at the time before dexmedetomidine infusion(T1), 5 minutes after loading dose(T2), 10 minutes after loading dose(T3), 1,3 minutes after anesthesia induction(T4,T5), just before endotracheal intubation(T6), endotracheal intubation(T7) and 1,3,5 minutes after endotracheal intubation(T8-10). Results Before anesthesia induction compared with T1, BIS of T2 、T3 in Group D was decreased significantly[from（92.6±2.5） to（73.2±1.9） and（70.1±2.3）](P<0.05 or 0.01), while BIS in group C were similar to the initial values; During anesthesia induction compared with T3 , BIS in both groups were decreased significantly ; During endotracheal intubation compared with T6 , BIS of T7（34.8±2.2） was increased significantly in Group C (P<0.05 or 0.01), while BIS in group C were similar to the initial values. Compared with group C, BIS decreased significantly at time T2～5、T7 in Group D (P<0.05 or 0.01). Before anesthesia induction compared with T1, ABP of T2 、T3 in Group D was increased slightly, HR of T2 、T3 was decreased significantly (P<0.05 or 0.01); During anesthesia induction compared with T3 , ABP and HR in both groups were decreased significantly ; Compared with T6 , DBP, MAP and HR of T7,T8 were increased slightly , that of T9,T10 were similar to the values in Group D. During endotracheal intubation, while ABP and HR of T7~T10 was increased significantly in group C (P<0.05 or 0.01). Compared with group C, SBP、MAP of T4~T10 and DBP of T3~T10 were increased significantly in Group D. HR of T2~T10 were decreased significantly in Group D(P<0.05 or 0.01). Conclusion Dexmedetomidine pretreatment significantly deepen anaesthetize, decreased BIS and alleviated haemodynamics reaction during anesthesia induction in patients undergoing valve replacement surgery with cardiopulmonary bypass. It is worthy of being recommended in clinical work．