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. A first pipe connects a hydrodynamic center of the retarder to a valve. A second, leakage pipe is disposed between a working medium outlet of the retarder and the valve. A third pipe is disposed between the valve and the working medium container. The valve is designed such that in a first position, a conducting connection is established between the working medium container and the leakage pipe to provide for a pumping operation of the retarder. When the valve is in a second position, a conducting connection is established between the working medium container and the first pipe for a braking operation of the retarder.