This paper describes an electrochemical method which permits us to transform solid metals (cobalt, iron or nickel) into nanoparticles. An electrolysis cell is made, the anode being a metal bar and the cathode a mercury layer. Magnetic nanoparticles are obtained in one step by electroreduction of mercury. Electrolysis is performed in an aqueous medium at pH above 6 in order to avoid the reduction of protons. The magnetic nanoparticles obtained are kept in mercury and can be recovered in an organic solvent.