Objective To evaluate the effect of dexmedetomidine administered locally on the median effective concentration ( EC50 ) of ropivacaine for paravertebral nerve block ( PVNB) . Methods Forty?eight ASA physical status Ⅰ or Ⅱ female patients, aged 20-64 yr, with body mass index<24 kg∕m2 , scheduled for elective unilateral segmental mastectomy under PVNB, were randomly divided into 2 groups ( n=24 each) using a random number table: ropivacaine group ( group R) and ropivacaine mixed with dexmedetomidine group ( group RD) . PVNB was performed at T4 on the operated side guided by ultrasound and nerve stimulator. Ropivacaine 20 ml and a mixture of ropivacaine and 20 μg dexmedetomidine 20 ml were injected locally in group R and group RD, respectively. The concentration of ropivacaine was determined by up?and?down sequential allocation. The initial ropivacaine concentration was set at 0. 35%, and the ratio between the two successive concentrations was 1. 2. The EC50 and 95%confidence interval of ropivacaine were calculated using Dixon?Massey method. Results The EC50 ( 95%confidence interval) of ropivacaine was 0.27% (0.23%-0.30%) and 0.22% (0.18%-0.25%) in group R and group RD, respectively. Compared with group R, the EC50 of ropivacaine was significantly decreased by 19% in group RD. Conclusion Small dose of dexmedetomidine administered locally can significantly enhance the efficacy of PVNB with ropivacaine.