PURPOSE: To separately control increase in the diameter of an excavated fluid hole extending through a jar and both upward and downward jarring actions.

CONSTITUTION: A double-acting hydraulic drilling jar 1 includes a mandrel 2 arranged in the housing 3 so as to be longitudinally slidable. A hammer 69 is disposed in the mandrel 2 to impart upward and downward jarring forces to the drill strings in the interaction with the anvil surfaces 64, 66 in the housing. A hydraulic valve arrangement permits the storage of large amount of static force before releasing the hammer 69 to strike the anvil surfaces with a great force. The hydraulic valve arrangement includes a tripping valve positioned to be actuated by a first pair of engaging surfaces in response to downward movement of the mandrel 2 in the housing 3 and a second pair of engaging surfaces in response to upward movement of the mandrel 2 in the housing 3. Thus, independent control over the upward and downward jarring action is achieved.