To evaluate the interaction between fentanyl and vecuronium, I have studied the recovery of train-of-four(TOF) responses after administration of vecwonium in 40 patients under enflurane-N2O inhalational anesthesia. Patients were allocated randomly to 2 groups of 20 patients each to receive: 3 ug/kg fentanyl(fentanyl group) and normal saline of same volume (control group) 10 min before administration of 0.1 mg/kg vecuronium. The times from initial administration of vecuronium to the first appearances of Tl, T2, T3, and T4 differed significantly between groups (p<0.01): 15.8±6.5 min, 21.4±7.2 min, 25.1±8.2 min, and 27.9±8.7 min in the control group, and 22.7±5.1 min, 28.4±5.3 min, 31.9±5.9 min, and 34.7±6.7 min in the fentanyl group, respectively. However, the T2-T4 recovery index did not differ significantly between groups: 5.9±3.1 min in the control group, and 6.3±3.0 min in the fentanyl group. It is concluded that fentanyl can slightly prolong the action duration of vecuronium, but not affect it during the recovery from muscle relaxation. (Korean J Anesthesiol 1995; 29: 364~367)