Abstract: An endoscopic lithotriptor including an elongated, flexible stone impacting member in the form of a wire whose distal end is brought against a stone to be broken and whose proximal end is adapted to be periodically struck by a hammer element. The hammer element is pneumatically driven by a linear jet of air causing it to swing through an arc about a pivot to impact an anvil affixed to the wire and oriented tangentially to the arc swept out by the hammer. The pneumatic chamber structure in which the hammer is pivotally mounted is designed to maximize the angular velocity reached by the hammer before impacting the anvil.