The Bain circuit system was first used by Bain and Spoerel in 1972. It is a type of nonrebreathing system which is a variation of the Mapleson D or E system. Its total length, diameter of the inner tube and diameter of the outer tube are 1.8m, 7mm and 22 mm respectively. It's CO₂ retension and fresh gas flow requirements were already studied by Bain in 1972. This study suggested that most patients were maintained in normocarbia when fresh gas inflow of 70 ml/kg was used under controlled ventilation. This was confirmed by Henville and Adams in 1976. If the respiratory depression did not .appear, normocarbia could be maintained with the same fresh gas inflow under spontaneous ventilation. The authors performed this study using the same fresh gas inflow in order to understand he problem of fresh gas requirement and CO₂ retension under controlled and spontaneous ventilation. The results of our study showed that the CO₂, retension was not seen in both groups and complications were not seen during the operation and postoperative period. The Bain circuit system was used satisfactorily at all age groups using low fresh gas flow. Especially in neurosurgery and obstetric anesthesia for maintaining a normal range of PaCO2, the Bain circuit system may be valuable.