During laparoscopic surgery with carbon dioxide (CO2) pneumoperitoneum, PaCO2 (arterial CO2 gas tension) and P(ET)O2 (end-tidal CO2 gas tension) will be affected by the durations of CO2 pneumo-peritoneum and the body positions. PaCO2 and P(ET)CO2 were investigated 5 minutes after induction of general anesthesia(control value), 10 minutes, 30 minutes and 60 minutes after CO2 gas insufflation, and 15 minutes after CO2 gas excretion. Seventy-two patients undergoing laparoscopic surgery under general anesthesia were allocated to two study groups: group I, laparoscopic appendectomy under the Trendelenburg position; group II, laparoscopic cholecystectomy under the reverse Trendelenburg position. In results, PaCO2 and P(ET)CO2 were significantly increased during laparoscopic surgery that associated with times of CO2 pneumoperitoneum. PaCO2 and P(ET)CO2 at 60 minutes after CO2 gas insufflation were increased from P(ET)CO2 control value 35.8±4.2 mmHg, P(ET)CO2 . control value 34.0±3.6 mmHg to P(ET)CO2 . 39.98.0 mmHg, P(ET)CO2 42.3±4.7 mmHg(p<0.05). PaCO2 and PO in group I were more increased compared with group II. PaCO and P(ET)CO2 in group I were increased from PaCO2 control value 35.9±4.8 mmHg, P(ET)CO2 control value 34.9±3.7 mmHg to PaCO2 45.7±2.5 mmHg, P(ET)CO2 48.0±3.6 mmHg(p<0.05), in group II from PaCO control value 35.7±3.2 mmHg, P(ET)CO2 control value 32.8±3.0 mmHg to PaCO2 38.4±8.3 mmHg, P(ET)CO2 40.4±3.2 mmHg(p<0.05). In conclusion, to minimize the risk of a carbon dioxide retension during laparoscopy especially under the Trendelenburg position, we recommend that ventilation should be adjusted to to the normal range of PaCO2 and P(ET)CO2.