Apparatus for the preparation of fluids intended for medical use is disclosed, including a source of pure water and at least one cartridge or other vessel containing a powder which is to be dissolved in the water for preparation of the desired fluid. The apparatus includes a container such that the water and a powder or powder concentrate can be mixed and a concentrate provided in the container, and a recirculation circuit for recirculation of the water or concentrate solution into the container for further mixing of the water and powder or powder concentrate to prepare a concentrate having a predetermined concentration.