Abstract: A ventilation system for a wind turbine generator system having a ventilation duct for conducting a cooling air that has been heated by waste heat from an air intake section of the ventilation duct to an air outlet section of the ventilation duct, along a flow path bounded by a duct wall which extends between the air intake section and the air outlet section, wherein in a sectional plane, preferably in every sectional plane that extends perpendicular to the flow path, the inside span of the ventilation duct can be adjusted by applying pressure to an outer boundary surface of the duct wall.