Abstract: The transmission comprises a hollow input member (1) and a coaxial output member (2) supported for rotation relative to the input member. A positive displacement pump comprises a cylinder block (12) secured to the input member and a rotor (16) which is journalled on an eccentric pin (17) and which drives the pump pistons (15). A positive displacement motor comprises a stator part (6) secured to the input member and a rotor part (5) secured to the output member. The motor cylinders (8) are supplied with hydraulic liquid from the pump through duct means (35, 37, 39) in a disc-shaped control member (22) journalled on the eccentric pin (17) and rotating together with the pump rotor (16). Return flow from the motor to the pump occurs through a space (28) which within the input member surrounds the control member (22) and which communicates with each pump cylinder (14) during the suction stroke of the associated pump piston (15).