Abstract: The invention concerns an automatic control system for the transfer of the drive power between a tractor (10) and a trailer (11), which system comprises a closed hydraulic circuit (43) that operates the drive wheels of the tractor (10) and an open hydraulic circuit (25) that operates the drive wheels (17a,17b) of the trailer. In the system, the closed hydraulic circuit (43) of the tractor (10) includes a pump (15) and a hydraulic motor (16) driven by the pump and, in a corresponding way, the open hydraulic circuit (25) of the trailer (11) comprises a motor or motors (21a,21b) driven by a pump (20). The control system comprises a line (42) that transmits a control pressure (P.sub.2) from the closed circuit (43) of the tractor (10) to the open circuit (25) of the trailer (11). When the resistance to traction increases, e.g. when driving up a steep hill, the pressure (P.sub.2) in the transmitting line (42) increases and, because of this pressure (P.sub.