*The integrity of a circle system and its valves should be checked by placing one breathing bag in the correct place for ventilating a patient and another breathing bag on the patient limb of the Y-piece (i.e. in place of the patient) and ventilating the system manually using an appropriate fresh gas flow and squeezing the primary and secondary bags alternatively, so that gas passes around the circle from one to the other. Inflation and deflation of the breathing bag, movement of any visible unidirectional valves, and the resistance and compliance of the system should all be assessed as “normal”. The function of the adjustable pressure limiting valve should also be checked by spilling some of the gas when both bags are compressed. This “two bag check” is a reliable way of detecting expiratory limb obstruction which is readily missed when less systematic checks of the integrity of the circuit are carried out.