ABSTRACT This paper presents results from a test programme on jars and jar accelerators. More than twenty jars in the range of 6.25" OD to 8" OD have been tested in a horizontal jarring rig built especially for this investigation. The test program comprises both single tool tests and full jarring tests where the jars are tested together with accelerators. In the single tool tests, the jars are extended and closed slowly to provide data on release forces, friction forces, jar strokes and delay times for hydraulic jars. In the full jarring tests the jars have been tested dynamically under realistic conditions. In these tests the dynamic friction is measured at realistic telescopic speeds and the strength of the jar is checked. The tests have revealed significant differences in the performance of various jars. In particular, dynamic friction forces vary widely between tools. The test results and their applications to downhole jarring are discussed.