An upstroke jar for use in downhole operations in an oil or gas well and for connection between a wireline and a tool comprises a casing defining an anvil and for connection to the tool, a hammer slidable axially within the casing for impacting with the anvil and for connection to the wireline, a hammer holder, a spring arrangement located between an abutment on the casing and an abutment on the hammer holder for compression on an operative movement of the hammer holder as a consequence of tension on the wireline and a connector releasable to free the hammer from the holder at the completion of the operative movement so that the freed hammer may impact with the anvil. The compression load on the spring present at the release is dependant on the extent of the operative movement from a rest position and the degree of compression of the spring means to obtain the operative movement. Adjustment of the release load present at the instant the hammer part is released by the connector is provided.