The invention relates to a method for anaesthetizing animals before slaughter, by the anesthetic action of gases or gas mixtures, operated in “batch” mode, in which the live animals, preferably put together in one or more cages, are placed in a lock chamber for treatment where the animals are brought into contact with anesthetic gas, following a cycle comprising several steps characterized by different contents of anesthetic gas, characterized in that at least one of the steps is carried out using gas recovered from the lock chamber and stored in at least one capacity (2, No. 1, No. 2 etc.) for temporary storage having a variable volume.