The invention concerns a drive unit having the following features: a motor (4.0), a hydrodynamic retarder (3.0) and at least one pump (1.0); at least one feed line for feeding operating medium to the retarder (3.0); at least one bypass line (3.1) which conveys operating medium past the retarder (3.0); and means (2.0) which are arranged upstream of the retarder (3.0) in the direction of flow and either feed the operating medium alternatively through the retarder (3.0) via the feed line or past the retarder (3.0) via a bypass line (3.1), or divide the total flow into at least two predetermined parts, a first predetermined part of the operating medium being fed through the retarder (3.0) via the feed line and a second predetermined part of the operating medium being conveyed past the retarder (3.0) via the first bypass line (3.1).
