BACKGROUND: During general anesthesia the adult patient's tidal volume is determined by patient's weight. Therefore patients with severe ascites, huge abdominal mass or pregnancy have considerable ventilatory change during the operation because the great change in their body weight, that might cause unwanted respiratory alkalosis or acidosis. The purpose of this study is to identify the effects of such changes in ventilation between pre-delivery and post-delivery periods during the cesarean section. METHODS: Eighteen 36weeks or older pregnant patients (ASA 1), scheduled for elective cesarean section under general anesthesia, are selected for the study. PhysioFlex was used for ventilation. Pre-induction maternal gas analysis were performed in all cases. They were randomly divided into two groups according to the ventilator setting for minute ventilation, one set by pre-pregnant weight (Group 1) and the other by pregnant (term) weight (Group 2). Blood gas analysis were performed 5 minutes after induction and 20 minutes after delivery. End-tidal CO2 and inspiratory airway pressure were also measured. RESULTS: All patients show mild hyperventilation with normal acid-base status at pre-induction period. There were no significant differences in arterial PCO2 value between two groups. Group 1 shows mild metabolic acidosis compared with group 2 (by pH at 20 minutes after delivery). CONCLUSION: Term(just prior to operation) body weight rather than pre-pregnant weight is preferable for ventilator setting in the case of cesarean section under general anesthesia.