One of the disadvantages of conventional circle absorber breathing circuits is that twisting between tile inspiratory and expiratory limbs sometimes occurs. In situations where access to the patient's airway is limited, such as during neurosurgical and head and neck operations, this disadvantage may lead to accidental disconnection or extubation. Another is that the temperature and humidity of inspired gas are usually less than desirable, predisposing to heal loss.1and drying of tracheal mucusa.2We have constructed a circuit that houses the inspiratory limb entirely within the lumen of a slightly larger expiratory limb. This makes it easy to handle and allows heat exchange between the cold inspired and warmer expired streams. Heat of condensation liberated by condensation of water on the outer surface of the inspiratory limb enhances vaporization of water droplets normally deposited on the inner surface of that limb, thereby increasing inspired humidity.