Objective To evaluate the effect of clonidine as an adjuvant of PCEA with morphine and ropivacaine in patients with essential hypertension and it' s effects on hemodynamics and plasma concentrations of endothelin-1 ( ET-1) and calcium gene-related peptide ( CGRP) . Methods Sixty ASAⅠ - Ⅱ patients of both sexes (24 males, 36 females) with a history of essential hypertension for 1-2 years were included in this study. The patients ranged in age from 45-72 yrs and in body weight 55-70 kg and were scheduled for elective pelvic surgery under combined general-epidural anesthesia. An epidural catheter was placed at T12-L1 or L1-2 interspace before induction of general anesthesia. Anesthesia was induced with fentanyl 4 ?g ? kg-1 , propofol 2 mg?kg-1 and vecuronium 0.1 mg?kg-1 and maintained with isoflurane and epidural analgesia. The patients received PCEA after operation. The PCEA regimen included a loading dose of 5 ml followed by background infusion at 2 ml?h-1 with an 1 ml bolus dose and a 15 min lockout interval. The PCEA solution contained morphine 2 mg + ropivacaine 75 mg in group A (n = 20); clonidine 150 ?g + morphine 2 mg + ropivacaine 75 mg in group B ( n = 20) ; clonidine 300 ?g + morphine 2 mg + ropivacaine 75 mg in group C ( n = 20) in 60 ml of normal saline. BP, HR, VAS pain score (0 = no pain, 10 worst pain) and Ramsay sedation score (1 = wide awake, 5-6 over sedated) were recorded one day before operation (T0), before induction of anesthesia (T1 ), 0, 5, 15, 30, 60 min, 2, 4, 8, 20, 24 h (T2-11 ) after PCEA was commenced. Blood samples were taken before induction of anesthesia (baseline) and 6 and 24 h after operation for determination of plasma concentrations of ET-1 and CGRP. The total number of button pressing (D1) and the number of actual delivery of bolus dose (D2) and the consumption of PCEA solution were also recorded. Results VAS pain score was significantly higher in group A than that in group B and C ( P