A drive unit includes a hydrodynamic retarder having a rotor paddle wheel, a stator paddle wheel, and a housing surrounding the rotor and stator wheels. The unit includes a coolant cycle system for an internal combustion engine, the coolant for this coolant cycle also being a working medium of the retarder. The drive unit further includes a working medium container for the coolant which is connected to the coolant cycle system. A connecting pipe is disposed between the working medium container and a hydrodynamic center of the retarder. A valve is disposed in the connecting pipe.