In a method for controlling an expiratory valve in a ventilator during expiration, including a stage in which the expiratory valve is opened almost completely for a first interval. Opening the expiratory valve almost completely results in a reduction in expiratory resistance. Thus in the first interval of flow or pressure in the expiratory part of the ventilator, a determination is made when the flow or pressure in the expiratory part meets a condition. A second interval is started when the flow in the expiratory part meets the condition and the expiratory valve is regulated during the second interval in order to attain a pre-set end pressure in the expiratory section.