A new catalytic procedure for atom-transfer radical addition (ATRA) and cyclization (ATRC) reactions is described. The combination of the ruthenium(III) complex [RuCl2-Cp*(PPh3)] (Cp*: pentamethylcyclopentadienyl) with magnesium allows these reactions to be performed under mild conditions with high efficiency. In most cases, the catalyst concentrations required are significantly lower than those used in previously reported procedures. It is suggested that magnesium acts as a reducing agent that generates and regenerates the catalytically active ruthenium(II) species. ne precatalyst [RuCl2Cp*(PPh3)] has been analyzed by X-ray crystallography.