Aim To determine the effective concentration of remifentanil in 50%essential hypertensive patients (EC_(50)) for blocking the sympathetic activation induced by desflurane.Methods Thirty essential hypertensive patients scheduled for elective surgery were enrolled in the study.The patients were inspired with desflurane 0.5 MAC after endotracheal intubation.Twenty minutes later,the first dose of remifentanil 4μg/L which was determined by up and down sequential experiment was given by target plasma concentration(TCI).Both the blood perssure(BP) and heart rate (HR) were recorded and averaged every 30 seconds of 3 minutes before or after fast inducting desflurane 1.5 MAC.If both the rate of average BP and average HR were increase 15%the next patient received remifentanil of a lower concentration; if both the rate of average BP and average HR were descrease 15%the concentration of remifentanil was increased in the next patients.Result Among the 30 essential hypertensive patients,five were excluded from the study because of uncertain efficacy.The results showed that the EC_(50) of remifentanil blocking the sympathetic activation induced by desflurane was 2.09μg/L(95%CI was 2.08μg/L～2.10μg/L).Conclusions The EC_(50) of remifentanil given by TCI which blocked the sympathetic activation induced by desflurane in essential hypertensive was 2.09μg/L determined by sequential experiment method.