We examine the rheological response of zeolite-based electrorheological (ER) suspensions in silicone oil, mineral oil, and N-(4-methoxybenzylidine)-4 butylaniline, a low molecular weight, nematic liquid crystal. The carriers were selected to explore the effects of fluid viscosity and dielectric permittivity on the ER properties of the suspensions. A particular form of the White-Metzner constitutive equation correctly reproduces the main, qualitative features of the experiments, with only three parameters. When used in conjunction with an equation describing rheometer dynamics, the rheological model satisfactorily captures artifacts caused by the coupling of inertia and elastic compliance of the instrument with the nonlinear material response.