The linear regulator problem is studied for systems with slow and fast modes. Using the descriptor variable method, the design of sub-optimal regulators for such systems is completely decomposed. It is shown that the sub-optimal regulator is composed of fast and slow regulators, and that both of these, mutually decoupled, can be designed independently. A relevant computational procedure is also presented. The procedure is applicable to designs of sub-optimal regulators with either finite or infinite terminal time, and for both singular and normal singularly perturbed systems.