The modifications of Ayre's T-piece system can be divided into three types. In the first there is no expiratory limb, in the second the capacity of the expiratory limb is greater than the tidal volume, and in the third the capacity of the expiratory limb is less than the tidal volume. On the basis of previous mathematical and laboratory investigations by other authors, the three types are discussed with reference to the resistance to respiration and minimum fresh gas flow required to prevent rebreathing and air dilution during both spontaneous and controlled ventilation. The resistance and the fresh gas flow requirements are related to the expiratory flow rates and respiratory flow patterns which occur in children. It is suggested that the most convenient system is a T-piece of 10 mm diameter with an expiratory limb greater than the tidal volume and a bag attached to the expiratory limb. A fresh gas flow of up to 2½–3 times the minute volume is required.