A method and an apparatus for setting the temporary lock-open mechanism incorporated in a safety valve employed in the subterranean well, the apparatus including an annular mandrel having an external, axially movable, radially expandable collet operated by an axially movable prong concentrically arranged within the mandrel housing.The temporary lock-open mechanism of the valve includes latch fingers which are adapted to engage the end of a plunger which opens the valve, a spring biased lock-open sleeve and release keys for holding the sleeve in a passive position.The valve is temporarily locked in an open position by introducing control line pressure into the valve for opening it and moving its lock-open mechanism to a position where the expandable collet of the apparatus can release keys that restrain the lock-open sleeve. The control line pressure is then bled off, thus allowing the spring biased lock-open sleeve to move to a position for camming the latch fingers into engagement with the end of the valve opening plunger.