Abstract: An apparatus for controlling the flow of gas to and from a patient in an anaesthesia system has first and second flow passages arranged in parallel between the patient and a reservoir bag. Selector valves allow multiple anaesthesia configurations involving no rebreathing or considerable rebreathing. A selector valve can be operated to connect the reservoir bag to either or both flow passages. A one way valve in the first passage, which communicates with a gas supply inlet, can be operated to prevent the back flow of gas from the patient into the passage. A one way valve in the second passage can selectively prevent the flow of gas in the second passage to the patient. The second passage has a selectively bypassable carbon dioxide absorbing fitter and a spill valve. The apparatus allows reconfiguration without disconnecting and reconnecting hoses.