A method for operating a drive train of a motor vehicle, which utilizes at least one permanent-magnet electric machine (EM) as a motor vehicle drive source, is provided. A transmission (G) with different gear ratios is arranged in a power path between the electric machine (EM) and driving wheels (DW) of the motor vehicle. A power converter (LE) is associated with the electric machine (EM) and can operate the electric machine (EM) in a field weakening condition. The method includes prematurely carrying out, delaying, or preventing a changeover of the gear step of the transmission (G) specified by a driving strategy depending on a temperature value of the electric machine (EM) in order to reduce, not increase, or increase with delay a rotational speed of the electric machine (EM). An electronic control unit (ECU) for carrying out the method and a motor vehicle with the control unit (ECU) are also provided.