A computer-controlled, closed-loop system for continuous infusion of muscle relaxants that allows the operator to choose either atracurium besylate or vecuronium bromide to provide any desired target level of neuromuscular blockade is described. This new system offers certain advantages over computer-controlled systems described previously for continuous infusion of muscle relaxants--that is, the option to choose either of two muscle relaxants to be infused and the inclusion of monitors to provide feedback about the dosage needed to produce a target level of neuromuscular blockade. The clinical performance of this system was tested in 36 patients who were 18 to 65 years old, were classified in American Society of Anesthesiologists physical status 1 or 2, and were undergoing elective orthopedic, abdominal, or thoracic procedures. In all patients, the control algorithm rapidly induced the target level of neuromuscular blockade and maintained that level of blockade at steady state with minimal oscillation. We conclude that the automatic feedback control system described can induce and precisely maintain any predetermined target level of neuromuscular blockade.