A hydraulic circuit of a servo-assisted mechanical transmission, which is provided with a number of hydraulic actuators provided with corresponding actuation chambers adapted to be filled with pressurized oil; the hydraulic circuit is provided with: a number of solenoid valves, each of which is coupled to a corresponding actuation chamber of a hydraulic actuator; a tank, which contains the oil at atmospheric pressure and is provided with an upper loading aperture; the cap, which closes the loading aperture of the tank and is provided with a through venting hole which puts the tank into communication with the external environment; a filtering element, which is coupled to the cap and is arranged under the venting hole; a hydraulic accumulator containing the pressurized oil; and an electric pump which draws from the tank and feeds the hydraulic accumulator.