The invention relates to a face mask arrangement for alternately administering a gas I containing nitrous oxide and a gas II devoid of nitrous oxide to a patient, comprising a face mask, two inlet flow lines I and II for gas I and a gas II, respectively, which flow lines end in a breathing interface, and an outlet flow line for evacuation of exhaled gas. The two inlet flow lines merge into a common inlet part. Each of the inlet flow lines I and II comprises an inlet valve I and II, each of which is capable of opening in response to inhalation and of closing in response to exhalation. The arrangement comprises a switching means enabling switching between inhalation through inlet flow line I and inlet flow line II depending on the patients immediate need of gas I or gas II. The invention relates also to a system comprises the face mask arrangement and an apparatus for collecting inhalation air from patients inhaling gas containing nitrous oxide and to the use of the face mask arrangement for administration of nitrous oxide to a patient via inhalation and collecting nitrous oxide from exhaled air of a patient during the administration.
