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, a valve spool arranged and movable in line with the valve plunger, a spring urging the valve spool and the valve plunger in opposite directions to each other for enabling the valve spool to produce a throttle pressure variable with the distance of movement of the valve plunger in the presence of a line pressure directed into the transmission throttle valve, and a plunger guide member arranged so that the throttle pressure developed in the valve is augmented to the level of the line pressure in the event a failure takes place in a mechanical linkage providing engagement between the valve plunger and the accelerator pedal of the vehicle.