Abstract: A motor/pump includes a cylindrically shaped rotor journaled within a similarly shaped housing to define an annular fluid passageway therebetween. The annular passageway is divided into a plurality of discrete fluid chambers by mobile blocking members which are mounted on the rotor near its periphery and stationary blocking members which extend outwardly from the housing to block fluid passage. Both types of blocking members are preferably free to rotate in place. Fluid inlets and outlets positioned around the annular chamber alternately pressurize and depressurize the chambers formed by the blocking members. The stationary blocking members are coupled to the rotor for synchronous rotation to allow passage of the mobile blocking members thereby. One preferred embodiment includes dampeners in the fluid inlets and outlets so that rotation of motor/pump can be controlled to provide the desired motor/pump characteristics.