Electro- (ER) and magnetorheological (MR) materials exhibit substantial increases in yield stress when subjected to strong electric or magnetic fields. The applied electric/magnetic field satisfies Laplace's equations in the ER/MR material. In annular geometries, the resulting spatial variation of the electric/magnetic field results in a yield stress inhomogeneity. The analysis of annular Poiseuille flow of ER/MR materials herein assumes a power-law relationship between the field and the yield stress, and includes the effects of the inhomogeneous yield stresses and hyperbolic shear stress distributions. The approximation of the solution to this problem by flow through an appropriately defined rectangular duct tin which the yield stress is constant and the shear stresses are linearly distributed) can be remarkably accurate, even for nonslender ducts.