A method of operating a drive-train of a motor vehicle which comprises a hybrid drive with a combustion engine and an electric machine, a clutch connected between the combustion engine and the electric machine, a transmission connected between the electric machine and a drive output, and either a primary retarder connected between the electric machine and the transmission or a secondary retarder connected between the transmission and the drive output. In order to warm up hydraulic oil, when the clutch between the combustion engine and the electric machine is disengaged and a transmission gear is engaged, a braking torque is produced by either the primary or secondary retarder, in the drive-train, and the electric machine is operated in a torque-controlled manner such that it delivers a torque equal to the sum of a target torque determined in accordance with the driver's wish and the braking torque produced by either the primary or secondary retarder.