In this paper, a closed-loop system to regulate plasma osmolality in patients with central diabetes insipidus is investigated. Using model identification techniques, we obtained a 3
-order LTI model of the renal/body fluid system from an 11th-order nonlinear system. A Smith Predictor and an
controller were developed. The effectiveness of the controller to reject a step disturbance in the plasma osmolality is examined. This paper shows the potential use of control theory in the context of central diabetes insipidus.