Abstract: A system for administering an anesthetic gas to a patient has a mask, means for supplying an anesthetic gas to the mask when the patient inhales, and an exhaust line having its proximal end connected to the mask and adapted to have its terminal end connected to a source of vacuum to exhaust gas from the mask when the patient exhales. A closed reservoir is in series with said exhaust line between the proximal and terminal ends thereof. The reservoir has an inlet and an outlet with a check valve adjacent to the inlet to prevent the flow of gas out of the reservoir through the inlet when the patient inhales. Advantageously, the reservoir check valve is a diaphragm valve and the face mask has at least one nozzle for scavenging gas escaping from the mask.