A transmission throttle valve for use in a hydraulic control system of an automotive automatic power transmission, comprising a valve plunger movable with the accelerator pedal of an automotive vehicle and, a valve spool engaging the valve plunger through a spring urging the valve plunger and the valve spool to axially move in opposite directions relative to each other for producing a fluid pressure variable with the displacement of the accelerator pedal, wherein the valve spool is formed with an axial bore open at both ends and has the valve plunger slidably passed through the bore while the valve plunger is formed with a laterally enlarged axial portion forming a variable-volume space in which the fluid pressure developed by the transmission throttle valve is to be admitted to urge the valve plunger to axially move against the force of the spring so as to save the driver's effort for depressing the accelerator pedal.