A PTO transmission system in a vehicle for transmitting power to a PTO shaft driving an implement comprises a planetary drive unit having first and second input and output coupled to the PTO shaft. The first and second input means are coupled to an output shaft of the vehicle engine and to an output shaft of a drive motor, respectively. A control system controls the transfer of power from a power source to the drive motor. The power source is a variable power generating source driven by the PTO shaft. The drive motor is configured and adapted to decrease or increase the rotational speed of the output of the planetary drive unit on an increase or decrease of the power transferred to it, respectively.