A respirator with an inhalation line (5) connected to a gas supply device (2) and with an exhalation line (7) connected to an exhalation device (3), with a Y piece (8) connecting the inhalation line (5) to the exhalation line (7), and with a patient connection (9) going out from the Y piece (8). Partial rebreathing of CO2-containing exhaled air is made possible in a simple manner by providing a switchover device (4) between the inhalation line (5) and the gas supply device (2) as well as between the exhalation line (7) and the exhalation device (3). The switchover device is designed such that the flow connection is established between the inhalation line (5) and the gas supply device (2) as well as the exhalation line (7) and the exhalation device (3) in a first switching position and there is at least a flow connection between the gas supply device (2) and the exhalation line (7) in the second switching position.