A system of regulating multiple anesthetic agents in a breathing circuit of an anesthetic breathing apparatus is disclosed. The system comprises regulating (110) a primary anesthetic agent added to said breathing circuit to a set desired value of the amount of said primary anesthetic agent, and regulating (120) a secondary anesthetic agent in said breathing circuit, to an amount of said secondary anesthetic agent that it is equal to or less than a defined threshold level, e.g. of the concentration, of the secondary anesthetic agent.