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Heart rate and respiratory patterns were monitored in ten ambulatory female patients undergoing elective laparoscopy. The patients were anesthetized with isoflurane-nitrous oxide. An index of cardiac vagal tone determined from the heart rate pattern by quantifying the amplitude of respiratory sinus arrhythmia was elevated over four 10-min periods: before induction of anesthesia; during maintenance of anesthesia; upon arrival in the recovery room; and 20–30 min later when the patient was fully conscious. All ten patients' vagal tones were lowest during maintenance of anesthesia. During the recovery periods vagal tone increased and approached the conscious level. On-line analysis of respiratory sinus arrhythmia may provide a physiological index of the level of anesthesia and the rate of recovery