Abstract: A well jar is characterized by inner and outer telescoped members held from relative displacement by a latching member having an extending portion thereon. Imposition of an axially directed triggering force exceeding a predetermined magnitude is transmitted against the biasing force of a resilient member through a force transmitting arrangement to a latch release member provided with an opening therein sized to receive the extending portion of the latching member. The release member rotates in a predetermined angular direction to come into registry with the opening therein to receive the extending portion of the latching member as the latching member displaces radially outwardly to release the inner member. Upon release, the inner member responds to the axially directed force to drive impact surfaces disposed on the inner and outer members into an impact-producing engagement.