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An anesthesia system which integrates closed-loop control of ventilation, oxygen, nitrous oxide, and anesthetic agent delivery into a closed breathing circuit is described. Breathing circuit volume is regulated by controlling the sum of oxygen and nitrous oxide flow, and oxygen concentration in the breathing circuit is regulated by controlling the ratio of oxygen flow to the sum of oxygen and nitrous oxide flow. End-tidal anesthetic agent concentration is regulated by controlling the agent delivery, and end-tidal carbon dioxide concentration is regulated by controlling the ventilation. After tuning the ventilation and anesthetic delivery controllers in preliminary trials in ten dogs, the system was tested and evaluated in five dogs. All control loops were stable and exhibited time responses to step changes in setpoint or external disturbances which were clinically acceptable. The system makes available the advantages of closed-system anesthesia without encumbering the anesthesiologist with the control tasks associated with the technique.