Abstract: A reciprocating hydraulic motor, for example for a rotary percussive drilling head, includes a piston and cylinder having a piston face exposed to the pressure in a compression chamber which is connected through ports controlled by the piston to a supply pressure at one end portion of its stroke, and to exhaust at an opposite end portion of the stroke, while between these end portions of the stroke is a compression/expansion travel during which the compression chamber is closed and the liquid in it alternately expands over a higher range of pressure and is compressed over a lower range of pressure.For returning the piston, in the direction in which liquid is compressed, one of two opposed faces of the piston is exposed to the pressure in a biasing chamber which communicates permanently with a constant pressure supply while the other communicates with the compression chamber.