Abstract: A method and device for preventing distribution of fire gases in a ventilating system that serves a number of fire compartments (15, 16), the ventilating air, during normal operating conditions, having a certain predetermined direction of flow at the inlet side and the outlet side of the fire compartments (15, 16). It is significant that when a predetermined lowest pressure drop is registered, for the ventilating air in the direction of flow, over a measuring spot at the inlet side or the outlet side, a blocking is effected of the flowing area of the ventilating air past the measuring spot.