Abstract: Improvements in down-hole mud actuated hammers are disclosed. According to its broadest aspect the invention is a down-hole mud actuated hammer for use in a drill string, which includes a housing with an upper end having means for connecting to the drill string. A throat is located within the housing which throat includes a main flow passage to allow high pressure drilling mud to pass therethrough. A piston is provided which is adapted to move axially within the housing means to thereby reciprocate between an up position and a down position. The piston is moved between the up and down position by a minor portion of the high pressure mud which portion passes from the main flow passage into at least one piston actuating chamber. This minor portion of mud is exhausted from the piston actuating chamber to a low pressure region out of the housing without being returned to the main flow passage.