This paper summarizes development of a quasi-linear theory for rectangular-type anti-rolling tank and presents results of the ensuing experimental investigation. Theoretical and experimental studies are compared. It was found that the initial theoretical method, although it reflects gross behavior of fluid in the tank and moments exerted on the ship therefrom, should be regarded as the foundation for a better understanding rather than as a design technique. Experimental findings indicate that the antiroll tank is a non-linear control element over its practical range of operation. Development of a nonlinear mathematical representation incorporating empirical results, as necessary, is recommended.