A hydrostatic transmission for a vehicle having a closed loop hydraulic system hydraulically interconnecting a variable displacement hydraulic pump and a hydraulic motor. The closed loop hydraulic system includes first and second conduits forming the closed loop.The first conduit is connected with a first relief valve having high and low setting pressure and the second conduit is connected with a second relief valve having high and low setting pressure. A hydraulic charge pump is connected with the closed loop hydraulic system through a charge conduit and a pilot-operated change over valve is disposed between the charge pump and the first and second relief valves in such a manner that when fluid pressure in the charge conduit exceeds a predetermined value fluid under pressure from the charge pump is introduced into the first and second relief valves to thereby change over the same to have high setting pressure and when the fluid pressure in the charge conduit becomes less than the predetermined value the first and second relief valves are connected with a tank to thereby change over the same to have low setting pressure.