Abstract: The present invention accordingly relates to hydraulic jars which include elongated telescopically arranged mandrel and housing members relatively movable between contracted and extended positions such members respectively having a hammer and an anvil with the anvil being spaced from the hammer when the members are in the contracted position and arranged for contract when the members are in the extended position. The jar includes hydraulic means for retarding movement of the mandrel relative to the housing member for a given time during a jarring stroke when the jar is tensioned. The hydraulic means includes a fluid-filled chamber having adjoining first and second bores, the first bore being of lesser diameter than the second bore and a piston on the mandrel member being receivable in the first bore with the piston having a slightly smaller diameter than the first bore to restrict the flow of fluid past the piston as it moves relative to the first bore during the jarring stroke.