The Bain circuit system was first used by bain and spoerel in 1972. It is a type of non-rebreathing system which is a variation of the Mapleson D or E system. Its total length, diameter of the inner tube and the outer tube are 1.8 m, 8 mm and 22 mm respectively. In this study the children and infants who weight 5~20 kg were selected, and during open heart surgery using bypass technique the respiration was controlled mechanically by using the bain circuit system with a fresh gas flow of 200 ml/kg/min. The minimum fresh fas flow was 2 L/min and the maximum fresh gas flow 3.5 L/min. The patients were divided into three groups according to body weight; group 1(5~10 kg of B.W.), group 2(10.1~15 kg of B.W.)and group 3(15.1~20 kg of B.W.). Each group was subdivided into the cyanotic heart disease group and the acyanotic. The results of our study showed that CO2 retention was not seen in either group. We observed that the Bain circuit system was very satisfactory and valuable in all groups during controlled ventilation, and this technique is also suitable for prolonged surgery(4~10 hrs) in infants and children.