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In order to evaluate the possible physiologic significance of intra- and postoperative hypotension, we monitored arterial blood pressure and heart rate continuously for 36 hr starting the night before and ending the morning after operation in 34 gynecologic patients. The lowest pressures that occurred during physiologic sleep were compared with the lowest arterial pressures that occurred during anesthesia without deliberate hypotension. Two values were calculated: a preoperative baseline pressure, i.e., the average value recorded during the second hour of monitoring on the night before surgery, and a preanesthetic standard pressure, i.e., the average value of 15 consecutive measurements made at 1-min intervals in the operating room before the induction of anesthesia. The results indicate that a systolic pressure 10% below the PAS during anesthesia does not constitute physiologically significant hypotension because this range resembles the range that occurs spontaneously during unmedicated sleep or sleep aided with a mild hypnotic. These physiologic nadirs in blood pressure are assumed to be tolerated well by the patient. Intraoperative pressures in elderly patients frequently drifted below sleep-associated levels of blood pressure and may, therefore, constitute physiologically significant hypotension.