In the filtrate and/or dialysate regeneration system, which is expected to miniaturize the artificial kidney, ammonium ion decomposed from urea by immobilized urease is removed competitively by ion-exchangers from coexisting cations. Since divalent cations such as Ca2+ and Mg2+ are more favorably exchanged than ammonium ion, this system needs supplementation of these cations and, thus, additional amount of ion-exchanger. To minimize these requirements, we utilized positively charged membrane to process cation-free filtrate in which urea is dialyzed. Positively charged membranes were tested in vitro to evaluate separation efficiency between urea and cations. Equilibrium adsorption of ammonium ion to ion-exchanger with or without co-existing cations demonstrates that this filtrate regeneration system can reduce the amount of ion-exchanger up to one-half to one-third of that of the conventional system. In an ex vivo experiment with a mongrel dog, blood urea nitrogen (BUN) concentration was maintained at the same level or up to 23% less than the initial value.