Electro-rheological (ER) and magneto-rheological (MR) materials exhibit variable stiffness and damping properties when subjected to strong electric or magnetic fields. While pre-yield visco-elasticity is a very important feature of these materials when subjected to oscillatory shears, the resultant stresses are primarily dissipative. Including an ER and MR device in a lumped-parameter structure (i.e., a building frame), results in large damping forces where the devices connect to the frame. Such structures are therefore not proportionally damped. This paper examines non-proportional damping in the context of ER and MR devices for controlling base-excited structures.