Abstract: An anaesthetic gas flow regulator includes an oxygen gas flow line connected to a supply of oxygen, an anaesthesia gas flow line connected to a supply of anaesthesia gas, controllable valves connected to the oxygen and anaesthesia gas flow lines, a patient rebreathing circle for delivering and exhausting a mixture of oxygen and anaesthesia gas to and from the patient to a common outlet means, an oxygen analyzer and a gas volume analyzer. The oxygen analyzer senses the oxygen concentration of the mixture of oxygen and anaesthesia gas in the rebreathing circle, and provides an oxygen signal when a minimum predetermined oxygen concentration is sensed by the oxygen analyzer. The volume sensor generates a volume signal indicative of a minimum volume threshold value in the rebreathing circle.