This paper presents a new magnetorheological (MR) cell design along with a study of the magnetic field, shear rate, and time/shear strain influences on the properties and behavior of a MR fluid tested for long periods of time. The MR cell was designed to adapt a commercially available rheometer to measure the rheological properties of the fluid. Overall characteristics of the designed MR cell output capability are provided. Constant shear rate tests, two hours in duration, have been performed at shear rates between and under magnetic field intensities up to . The rheological measurements indicated that over time the fluid’s shear stress magnitude decreases until it reaches a steady state. The time required to reach the steady state depends on both the magnetic field strength and the shear rate. The higher the field and the smaller the shear rate the shorter the time for the steady state to be reached.