Abstract: A jar mechanism comprises a housing (22) having a fluid chamber (9) therein, a piston (5) encircled by an annular metering sleeve (19) and movably mounted in the fluid chamber for movement between a first position and a second position, and a jar member (2) moveably mounted in the housing such that a pull or push force exerted on the jar member moves the piston from the first position to the second position within the fluid chamber against the resistance of the fluid, and the action of the pull or push force exerted on the jar member actuates a release device. The jar member is releasably coupled to the piston by the release device (3,4) such that, when the piston is in the first position in the fluid chamber, the jar member is coupled to the piston by the release device for movement therewith. Actuation of the release device enables the jar member to be uncoupled from the piston.