Abstract: A ventilating apparatus includes a closed breathing loop (3) which is supplied with the gases needed for ventilation via a metering unit (9). The ventilating apparatus is improved in such a manner that the respiratory gas can be metered during the inspiration phase without affecting the respiratory gas stroke volume supplied to the patient. The ventilating apparatus includes a flow-measuring device (14) for detecting a gas volume (V.sub.F) which is metered into the closed breathing loop (3) during the inspiration phase. A subtraction element (16) forms a difference volume (V.sub.T -V.sub.F) between the pump volume (V.sub.T) and the gas volume (V.sub.F). A switch (21) permits utilizing the difference volume (V.sub.T -V.sub.F) as a new adjusting value for the pump volume of the respiratory gas pump unit (2).