Objective To determine the plasma concentrations and pharmacodynamics of fentanyl and nerphine used in postoperative epidural or intravenous patient-controlled analgesia (PCEA,PCIA) .Methods Sixty ASA Ⅰ - Ⅱ patients (36 male, 24 female), aged 18-25 yr undergoing elective major operation were randomly divided into 3 groups : group Ⅰ received PCEA with 0.13 % bupivacaine + fentanyl 2?g?ml-1 (n = 20) ; groupⅡ received PCEA with 0.13 % bupivacaine + morphine 0.08 mg? ml-1 ( n = 20); group Ⅲ received PCIA with morphine 0.5 mg ? ml-1 ( n = 20). In group Ⅰ and Ⅱ the background infusion rate was 4 ml ? h-1 , PCA bolus dose 2 ml and lock-out interval 20 min, while in group Ⅲ the back ground infusion rate was 1 ml?h-1 , PCA bolus dose 2 ml and lock-out interval 6 min. PCA was maintained for 48 h in all three groups. The vital signs, analgesic effect (VAS, VRS) and side-effects were recorded and venous blood samples were taken for determination of plasma fentanyl and morphine concentrations at 4 h, 24 h and 48 h after PCA was commenced. Results The demographic data were comparable among the three groups. There was no significant difference in MAP, HR and RR during PCA among the three groups. The analgesia was satisfactory in all three groups and no other analgesic was used during PCA. The rate of excellent analgesia ranged between 80%-85 % . The incidences of side-effects were higher in group Ⅱ and Ⅲ as compared with those in group Ⅰ . The volume of epidural PCA solution administered in 48 h was significantly larger in group Ⅰ than that in group Ⅱ( P