A PTO transmission system in a vehicle for transmitting power to a PTO shaft driving an implement comprises a planetary drive unit (3) having first and second input means (2, 5) and output means (4) coupled to the PTO shaft (8). The first and second input means are coupled to an output shaft of the vehicle engine (1) and to an output shaft (5) of drive motor means (7), respectively. Control means (11) control the transfer of power from a power source (4) to said drive motor means (7). The power source is a variable power generating means (6) driven by said PTO shaft. The drive motor means are configured and adapted to decrease or increase the rotational speed of the output means of the planetary drive unit on an increase or decrease of the power transferred to it, respectively.