Abstract: A method and an arrangement in connection with vaporizing an anaesthetic, in which an anaesthetic, vaporized in a vaporizer is mixed with fresh gas and delivered to be inhaled by a patient. In order to achieve a reliable operation, the prevailing ambient air pressure is measured, the measured air pressure is compared with a reference pressure and when the prevailing ambient air pressure deviates from the reference pressure, the anaesthetic agent concentration of the fresh gas/anaesthetic mixture to be passed to the patient is adjusted in relation to the variation in the ambient external air pressure.