A microcomputer based control system has been developed to automatically induce and maintain neuromuscular block during surgery. The system repeatedly adjusts the infusion rate of a muscle relaxant, succinylcholine, based on the evoked, rectified, and integrated electromyogram (EMG). The system was tested in 12 patients for a 30-min infusion period with a setpoint of 80% depression of the evoked, rectified, and integrated thenar EMG. The mean time to reach the setpoint for 10 of the patients was 5.5 (±1.87 SD) min and the mean time for 95% recovery after infusion was stopped was 5.4 (±0.83 SD) min. Average overshoot was 9.9% of the baseline (±3.1% SD), and the average time within ±10% of the setpoint was 22.1 min (±7.26 SD). The total dose of succinylcholine for these 10 patients ranged from 1.21 to 3.77 mg/kg with a mean of 1.92 mg/kg. The other two patients were relatively insensitive to the drug and the controller was unable to bring the response to the set-point due to a ceiling placed on infusion rate by the control algorithm. © 1985 Pergamon Press Ltd.