In this study we attempted to define the minimal dosage of alfentanil (AF) needed in combination with nitrous oxide to provide satisfactory anesthetic conditions for lower abdominal gynecologic surgery. General anesthesia was induced in 12 women with AF (150 micrograms X kg-1) and 66% N2O in O2. An infusion of AF was started immediately after the AF induction dose and was varied between 25-150 micrograms X kg-1 X hr-1 as indicated by the patient's responses to stimulation during operations lasting 208 +/- 22 (SEM) min. Small bolus doses of AF (7 micrograms X kg-1) were administered to rapidly suppress precisely defined somatic, hemodynamic, and other sympathetic responses to stimulation. With one exception, all responses in all patients were controlled rapidly by increments of AF. The mean dosages of AF needed during different stages of surgery are reported. The AF infusion was stopped 16.2 +/- 1.2 min before discontinuing N2O. Recovery of consciousness along with satisfactory spontaneous ventilation occurred promptly after completion of the operation (4.0 +/- 0.5 min after N2O; 20.3 +/- 1.4 min after stopping AF infusion). This study demonstrates the feasibility of maintaining general anesthesia with N2O and a continuous AF infusion at a rate varied according to the patient's responses and allowing for prompt recovery of consciousness and satisfactory spontaneous ventilation at the conclusion of operations lasting as long as 5 hr.