A method for controlling the atmosphere in a protective gas chamber for the continuous treatment of metal strips. A metal strip is guided into and out of the protective gas chamber via locks. At least one lock has at least two sealing elements for the metal strip which runs through it, with the result that a sealed chamber is formed between the two sealing elements. The gas pressure (P2, PD) is measured in the protective gas chamber and in the sealed chamber of the lock and the pressure (PD) in the sealed chamber is regulated, to be precise in such a way that, during operation, the differential pressure between the protective gas chamber and the sealed chamber is kept as far as possible to an optimum value.