Objective To investigate the effects of different ventilation modes on PaCO 2 and PaO 2 during laparoscopic surgery in the Trendelenburg position.Methods Forty cases of laparoscopic resection of rectal cancer were randomly divided into group A (n =20)and group B (n =20).Under general anesthesia,all the cases were in volume-controlled ventilation mode (VCV,tidal volume=10 ml/kg)before pneumoperitoneum.After ventilating in VCV mode for 20 minutes with a lower tidal volume (8 ml/kg),group A was converted to PCV mode for 20 minutes and then back to the VCV mode for 20 minutes again.Next we switched to PCV combined with PEEP (5 cm H 2 O)mode (PCV+PEEP)for 20 minutes.In group B,we only alternated PCV with PCV combined with PEEP.Arte-rial blood-gas analysis was obtained at each time when ventilating mode changed.Results In both group A and B,PaO 2 in VCV mode was less than that in PCV mode and PCV+PEEP mode (P <0.05),PaO 2 in PCV mode was also less than that in mode PCV+PEEP mode (P <0.05).PaCO 2 in PCV and PCV combined with PEEP mode was less than that in VCV mode (P <0.05 ),and there was no difference of PaCO 2 in PCV mode and PCV+PEEP mode.The pH value in VCV mode was less than that in PCV mode and PCV+PEEP mode (P < 0.05).There was no difference in pH value between PCV mode and PCV+PEEP mode.Conclusion PCV combined PEEP mode is beneficial ei-ther in increasing or decreasing of PaCO 2 during laparoscopic surgery in the Trendelenburg position comparing with single VCV mode or PCV mode.