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Use of a closed rebreathing circuit for anesthesia delivery offers several advantages: conservation of anesthetic agent, natural heating and humidification of inspired gases, less pollution, and improved monitoring. However, the technique requires careful control of the fresh gas and anesthetic delivery. An anesthesia delivery system has been developed which automatically controls fresh gas delivery, anesthetic delivery, and ventilation in order to regulate circuit volume, oxygen concentration, end-tidal anesthetic concentration, and end-tidal PCO2. This system makes available the advantages of closed-circuit anesthesia without encumbering the anesthesiologist with its more demanding control tasks. The system has undergone clinical testing in adult surgical patients. Maintenance of circuit volume, oxygen concentration, end-tidal anesthetic concentration, and end-tidal PCO2 was achieved by the system in all patients but the first. The anesthetic delivery controller required tuning on the first patient. No further tuning was required for any of the other patients or controllers. During abdominal surgery, the end-tidal bellows position measurement and end-tidal anesthetic concentration measurement increased in variability. The cause of the variability and its elimination are discussed in the article.