Abstract: Apparatus for releasing a cementing plug. A first cylinder mounted on a plug container includes a plunger received therein which is longitudinally moveable into the plug container for supporting a cementing plug. A second cylinder received over the first cylinder defines an annular chamber between the cylinders. An annular sleeve is sealingly and slidingly received within the chamber and is slidable between a first position toward the plug container and a second position away from the plug container. The first cylinder includes a slot formed in one side thereof and the plunger includes a recess formed on the radially outer surface thereof. A lock dog is received in the slot and is urged into the plunger recess when the annular sleeve is in the first sleeve position and is withdrawn from the recess when the annular sleeve is in the second sleeve position.