BACKGROUND: The present study was performed to elicit what effect nalbuphine would have on isoflurane MAC (minimum alveolar concentration) in the patients undergoing lower abdominal surgery. METHODS: Sixty-two women were randomly allocated to one of five study groups to receive an intravenous injection of no nalbuphine (group I), 0.25 mg/kg (group II), 0.5 mg/kg (group III), 1.0 mg/kg (group IV), 1.5 mg/kg (group V). Anesthesia and tracheal intubation were induced with propofol 2 mg/kg, succinylcholine 1 mg/kg. Patients were inhaled at a preset end-tidal concentration of isoflurane, which was maintained for 20 min. Response to skin incision, movement or no movement, was determined 30 minutes after nalbuphine injection. The isoflurane concentration of the next patient in the same group moved up or down in steps of 0.1~0.3%, according to the previous patient's response. MAC was determined using the "up-down" method and logistic regression. RESULTS: The MAC's of isoflurane were 1.09 vol% end-tidal in the control group, 0.89 vol% in group II, 0.65 vol% in group III, 0.55 vol% in group IV, and 0.51 vol% in group V. CONCLUSIONS: It would be suggested that nalbuphine dose-dependently reduce the isoflurane MAC, and have ceiling effect on the reduction of isoflurane MAC.