Abstract: A device for preparing a radioactive water solution to be infused in a patient includes a reaction chamber (13) in which radioactive water vapour is formed by the catalysed reaction of oxygen gas containing oxygen-15 and hydrogen gas. A diffusion chamber (14) is provided which allows the radioactive water to penetrate, but which prevents the penetration of gasses. Tubes (26) and valves (16, 18) direct a sterile saline solution to the diffusion chamber (14), and then direct the saline solution containing radioactive water out from the diffusion chamber to a patient, or to a decay coil (22) being a part of the device. A measuring instrument is provided for measuring the radioactivity of the radioactive solution. The device is characterized in that the diffusion chamber (14), the tubes (26) the valves (16, 18), the radioactivity measuring instrument (17), and preferably also the reaction chamber 913), are mounted in the same frame 50, whereby they form a separate unit i.e.