Eleven myasthenia gravis and seven control patients were studied during N2O/O2-fentanyl anesthesia to determine the ED50, ED95, and maintenance requirement of vecuronium using both mechanomyography and electromyography. The ED95of vecuronium was 17 (range 8–34) μg/kg in patients with myasthenia gravis, and this was significantly related to patient's acetylcholine receptor antibody titer (r= −0.75,P< 0.01). The average ED95value was 250% greater in control than in myasthenic patients (P< 0.01). The hourly requirement of vecuronium to maintain an 80–90% neuromuscular blockade was 38 ± 10 μg/kg in myasthenic and 120 ± 27 μg/kg in control patients (P< 0.001). When these requirements were related to individual ED95doses, they were comparable indicating similar time durations of effect of vecuronium following an equipotent dose in myasthenic and in control patients.