A positive displacement fluid transfer device is disclosed which may be operated either as a fluid pump, compressor or motor with no modification. The device is based mechanically upon axial cylinder and plunger hydraulic devices using a swash plate to reciprocate the plunger or plungers, but further includes a gear train to cause the plunger or plungers to simultaneously rotate as they reciprocate within the cylinders. Reliefs in the sides of the plungers periodically rotate into alignment with inlet and outlet ports to provide a fluid flow path. One manifold of the device is wholly contained within the bounds of the other, thus providing a compact system with a relatively direct fluid flow path; a special case provides for concentric inlet and outlet minifolds. Other improvements are provided, such as an internal pressure relief bypass between the inlet and outlet manifolds and utility passages which provide for cooling, lubrication and fluid leakage control past the plungers. The direction of rotation of the device is easily reversible either by reversing the inlet and outlet flow or by revising the positions of the plunger gears relative to the central gear.