Anaesthetic agent metering device for adjusting an anaesthetic agent concentration in a gas mixture to be provided to an anaesthetic breathing circuit, having - a first gas branch which is connectable to a breathing gas supply and is designed to conduct a breathing gas through an anaesthetic agent evaporator to a mixer unit, - a second gas branch which is connectable to the breathing gas supply and is designed to conduct the breathing gas directly to the mixer unit, - the anaesthetic agent evaporator which is arranged and designed to enrich the breathing gas in the first gas branch with an anaesthetic agent, - the mixer unit which is arranged and designed to mix the breathing gas, which is enriched with the anaesthetic agent, from the first gas branch with the breathing gas from the second gas branch and to provide the resulting gas mixture to an outlet of the anaesthetic agent metering device, - a first gas-flow-measuring unit which is designed to measure a first gas flow and to output a corresponding first gas flow signal, - a proportional valve which is arranged in the second gas branch and is designed to receive a valve control signal and to adjust a valve opening in accordance with the valve control signal, - a pressure-measuring unit which is arranged and designed to measure a gas pressure within the second gas branch and to output a corresponding valve pressure signal, and - a control unit which is designed to receive the valve pressure signal, the first gas flow signal and a second gas flow signal and, depending on the first and second gas flow signal (152, 158), to determine a first gas branch flow in the first gas branch and a second gas branch flow in the second gas branch, wherein the control unit is furthermore designed to receive a desired concentration signal which indicates a predetermined desired concentration of the anaesthetic agent at the outlet, and, on the basis of the predetermined desired concentration and the first and second gas branch flow (159, 153), to determine a desired chamber pressure of an evaporator chamber of the anaesthetic agent evaporator, wherein the control unit is furthermore designed to determine, depending on the desired chamber pressure and the valve pressure signal, a desired valve opening of the proportional valve, wherein the desired valve opening is determined in such a manner that, through the desired valve opening of the proportional valve, the desired chamber pressure within the anaesthetic agent evaporator in the first gas branch is reached, and wherein the control unit is furthermore designed to output the valve control signal indicating the desired valve opening to the proportional valve. - a first gas branch which is connectable to a breathing gas supply and is designed to conduct a breathing gas through an anaesthetic agent evaporator to a mixer unit, - a second gas branch which is connectable to the breathing gas supply and is designed to conduct the breathing gas directly to the mixer unit, - the anaesthetic agent evaporator which is arranged and designed to enrich the breathing gas in the first gas branch with an anaesthetic agent, - the mixer unit which is arranged and designed to mix the breathing gas, which is enriched with the anaesthetic agent, from the first gas branch with the breathing gas from the second gas branch and to provide the resulting gas mixture to an outlet of the anaesthetic agent metering device, - a first gas-flow-measuring unit which is designed to measure a first gas flow and to output a corresponding first gas flow signal, - a proportional valve which is arranged in the second gas branch and is designed to receive a valve control signal and to adjust a valve opening in accordance with the valve control signal, - a pressure-measuring unit which is arranged and designed to measure a gas pressure within the second gas branch and to output a corresponding valve pressure signal, and - a control unit which is designed to receive the valve pressure signal, the first gas flow signal and a second gas flow signal and, depending on the first and second gas flow signal (152, 158), to determine a first gas branch flow in the first gas branch and a second gas branch flow in the second gas branch, wherein the control unit is furthermore designed to receive a desired concentration signal which indicates a predetermined desired concentration of the anaesthetic agent at the outlet, and, on the basis of the predetermined desired concentration and the first and second gas branch flow (159, 153), to determine a desired chamber pressure of an evaporator chamber of the anaesthetic agent evaporator, wherein the control unit is furthermore designed to determine, depending on the desired chamber pressure and the valve pressure signal, a desired valve opening of the proportional valve, wherein the desired valve opening is determined in such a manner that, through the desired valve opening of the proportional valve, the desired chamber pressure within the anaesthetic agent evaporator in the first gas branch is reached, and wherein the control unit is furthermore designed to output the valve control signal indicating the desired valve opening to the proportional valve.