The electrorheological properties of well-characterized side-chain liquid-crystalline polysiloxanes with different backbone and spacer lengths were studied in two nematic solvents : 4’-(pentyloxy)4-biphenylcarbonitrile (50CB), which has positive dielectric anisotropy, and N-(4-methoxybenzylidene)4-butylaniline (MBBA), which has negative dielectric anisotropy. Specifically, we measured the steady-shear viscosities, eta(off) and eta(on), in the absence and presence, respectively, of a saturation electric field. For 50CB solutions, the electrorheological (ER) effect is positive, i.e., eta(on) > eta(off). For the intrinsic viscosities, we find [mu(off)] >> [eta(on)], and [eta(off)] shows a strong dependence on molecular weight ([eta(off)] similar to M-0.4) and spacer length, whereas [eta(on)] is insensitive to the change of molecular weight and spacer length. In contrast, the ER effect of MBBA solutions is very small and slightly negative, i.e., eta(on) < eta(off). In addition, [eta(off)] and [eta(on)] are of comparable magnitude and each shows a strong dependence on molecular weight ([eta(off)] similar to [eta(on)] similar to M-0.3). These distinctive patterns of ER behavior can be explained by a hydrodynamic model which assumes the side-chain liquid-crystalline polysiloxane has an asymmetric conformation, such that the root-mean-square end-to-end distance parallel to the director, R-parallel to, is different from that perpendicular to the director, R-perpendicular to. R-parallel to is aligned along the shear gradient in 50CB when the field is on but is tilted toward the now direction with the field off. In MBBA, R-parallel to is oriented perpendicular to the shear gradient both with the field on and with the field off.