A method improved for the assay of glycogen synthase(GS) is proposed in the present study. GS activity in NIDDM rat liver is determined by the solid phase filter paper method in comparison with washing method reported in the literatures. There is no obvious difference in the determination of radiation counts between the two methods. This method has been employed in the determination of GS activity of the diabetic rat liver, which exhibited more decrease then the normal rat liver. Therefore, the solid phase filter paper method is simple, convenient for the assay of GS activity and shows no tendency to be polluted.