KKAy mouse, a genetic animal model of diabetes mellitus, is characterized by not only hyperglycemia, but also polydipsia and polyuria like human diabetes mellitus. In this study, we investigated the effects of Byakko-ka-ninjin-to (BKN), Bofu-tsusho-san (BOF) and Gorei-san (GRE) on blood glucose level, water intake and urine volume in KKAy mice. All these Kampo medicines were administered to the mice as the food admixture containing 1.5% or 4.5% of weight of food for 4 weeks. BKN at 1.5% and 4.5% decreased water intake without affecting blood glucose level. BOF at 1.5% showed the same effects as BKN did, but it decreased both water intake and blood glucose level at 4.5%. Furthermore, BOF at 4.5% obviously, but BKN at 4.5% slightly, decreased urine volume. However, GRE at 4.5% did not show any effects on blood glucose level, water intake and urine volume. From these results, BKN is confirmed to be effective against the polydipsia accompanied by diabetes mellitus, while BOF is suggested to be effective against not only polydipsia, but also polyuria and hyperglycemia in diabetes mellitus.