Abstract: A reversible gerotor pump having unidirectional fluid flow therefrom dictated by a movable eccentric ring member 30 for positioning the rotor 46 of the pump gear-set 44 in either one of two axially eccentric positions depending upon the direction of rotation of the drive shaft 20 to the pump. Movement of the eccentric ring member from one such position to the other position upon a reversal of direction of the drive shaft is caused by check valve means 70 associated with the pump inlet 68 so that, if reverse rotation of the drive shaft momentarily causes reverse flow of the pumped fluid (e.g., causes the fluid to exit through the inlet) the check valve closes and establishes a hydraulic lock between the gear-set and forces the eccentric ring member to move from the one position to the other eccentric position whereupon the fluid is once again caused to be discharged through the normal pump outlet 64 and enter through the inlet.