A method of ultrafiltration and an artificial kidney therefor, in which a membrane which is capable of simultaneous ultrafiltration and dialysis of blood is arranged to form a portion of the wall of a constant volume, closed loop vessel. A dialysis liquid is circulated in the vessel, while blood is passed on the opposite face of the membrane, and a fraction of the dialysis liquid is withdrawn from the vessel at a predetermined rate. The remainder of the dialysis liquid is kept at a constant temperature and is regenerated.