Objective To observe the effects of three doses of dexmedetomidine hydrochloride on stress and myalgia in patients before conventional modified electroconvulsive therapy (MECT).Methods Seventy-nine patients scheduled for the first time of MECT with schizophrenia,depression or mania patients,38 males and 41 females,aged 18-65 years,ASA physical status Ⅰ or Ⅱ,were randomly divided into four groups.The patients received intravenous drip infusion of dexmedetomidine 0.4 μg/kg (group D1,n=20),0.7μg/kg (group D2,n=19),1.0 μg/kg (group D3,n=20) and saline 20 ml (group N,n =20) within 10 min before conventional MECT treatment.The mean arterial pressure (MAP) and heart rate (HR) were recorded before anesthesia (T0),drug pump (dexmedetomidine or saline) after 10 min (T1),30 s after MECT shock (T2) and 5 min after MECT shock (T3);the blood glucose and the concentration of serum cortisol (Cor) were recorded at T0 and T2;visual analogue scores (VAS) 30 min after electric shock (T1) and 6 h after shock (T5),the recovery time,recovery time of spontaneous breathing were also recorded.Results Compared with group N,the concentration of Cor and MAP at T2 were decreased significantly,the HR was increased significantly in groups D1-D3;the blood glucose at T2 were decreased significantly in groups D2,D3 (P＜0.05).The degree in group D3 was larger than that in group D2,and the degree in group D2 was larger than that in group D1 (P＜0.05).Compared with group N,the VAS score at T5 were decreased significantly in groups D1-D3;the recovery time was prolonged significantly in groups D2,D3.The degree in group D3 was larger than that in group D2,and the degree in group D2 was larger than that in group D1 (P＜0.05).The spontaneous breathing recovery time had no difference in three groups.Conclusion Dexmedetomidine pretreatment before modified electroconvulsive therapy can dose-dependently reduce stress response and treatment related myalgia and prolong the recovery time.