Abstract: Disclosed is releasable plugging apparatus for selectively closing a tubular member to permit fluid pressure buildup therein. When the fluid pressure is then decreased, a sealing device of the apparatus may be released, and the tubular member subsequently opened. Holding apparatus secures the sealing device in place until a compressed spring is permitted to move a piston so as to permit the holding apparatus to release the sealing device. A locking device prevents such movement by the piston until the pressure buildup first moves the piston in the opposite direction to release the locking device and to further compress the spring. The tubular member is thus not unplugged until the fluid pressure is decreased, thereby avoiding a large pressure pulse which might result if the sealing device were released with the fluid pressure at a high value. The embodiments described are particularly applicable for use with hydraulically-operated well tools, such as well packers.