An anesthesia system with an anesthetic evaporator can be operated even in case of a defect, particularly in case of a power outage. The anesthetic evaporator (10) and a valve arranged upstream of the anesthetic evaporator (10) are provided, whereby the gas flow through the valve (8) is conveyed completely or partially through the anesthetic evaporator (10) or a bypass line (9) past the anesthetic evaporator (10) in case of operation in accordance with the regulations, while the gas flow takes place only through the anesthetic evaporator (10) in case of a defect.