A hydraulic circuit for the transmissions of industrial and agricultural vehicles of the type comprising an internal combustion engine, a hydraulic transmission (2, 7, 8) connected to the hydraulic circuit, a hydraulically operated auxiliary unit and an output shaft driven by the internal combustion engine which is capable of providing a useful power to drive other working components, said circuit comprising: a. a feed pump operating on a working fluid of the hydraulic circuit and connectable to the internal combustion engine of the vehicle in order to be driven; b. a delivery section connectable to a hydraulic transmission (2, 7, 8) of the vehicle, said transmission comprising a lubrication circuit and/or a torque converter of the vehicle; c. an auxiliary uses section connectable to the hydraulically operated auxiliary unit for providing the working fluid at a working pressure to the hydraulically operated auxiliary unit; d. a main pressure regulator (3; 3') capable of bringing about a first variation in pressure (Δp1) in an operating fluid in the circuit through an inlet section connected to the pump and an outlet section connected to the delivery section; e. a first regulating section (33; 33') connecting said main pressure regulator (3; 3') to a switching device (5; 5') and to which a first regulating pressure is associated, said switching device (5; 5') being capable of alternately opening or closing a connecting branch (33a; 33a') to said regulating section (33; 33') in such a way as to modify the first regulating pressure and regulate a first pressure variation (Δp1); f. a maximum pressure regulator (4; 4') connected to the delivery section which can bring about a second pressure variation (Δp2) in the operating fluid between the outlet section and a discharge section; wherein said auxiliary uses section and said delivery section are positioned respectively upstream and downstream said main pressure regulator (3; 3'), wherein said switching device comprises a switching valve which can selectively close off the regulating section and place it in connection with a second regulating section connected to said outlet section and a third regulating section connected to a pressure limiting valve respectively. a. a feed pump operating on a working fluid of the hydraulic circuit and connectable to the internal combustion engine of the vehicle in order to be driven; b. a delivery section connectable to a hydraulic transmission (2, 7, 8) of the vehicle, said transmission comprising a lubrication circuit and/or a torque converter of the vehicle; c. an auxiliary uses section connectable to the hydraulically operated auxiliary unit for providing the working fluid at a working pressure to the hydraulically operated auxiliary unit; d. a main pressure regulator (3; 3') capable of bringing about a first variation in pressure (Δp1) in an operating fluid in the circuit through an inlet section connected to the pump and an outlet section connected to the delivery section; e. a first regulating section (33; 33') connecting said main pressure regulator (3; 3') to a switching device (5; 5') and to which a first regulating pressure is associated, said switching device (5; 5') being capable of alternately opening or closing a connecting branch (33a; 33a') to said regulating section (33; 33') in such a way as to modify the first regulating pressure and regulate a first pressure variation (Δp1); f. a maximum pressure regulator (4; 4') connected to the delivery section which can bring about a second pressure variation (Δp2) in the operating fluid between the outlet section and a discharge section;