The effect of oral Mg-supplementation as prophylaxis against noise-induced hearing loss was tested in a placebo-controlled double blind study involving 320 voluntary subjects during a 2-month period of military training. The hearing thresholds of all subjects were checked and only persons with normal hearing were accepted. Before and after the 2-month training, blood samples were collected and Mg was analysed in serum, erythrocytes and lymphocytes. Seven days after the last exposure to firearm noise, the audiograms of all test subjects were checked and permanent threshold shifts (PTS) were determined. The total group received a drink containing either 4g Mg granulate verum (6.7 mmol Mg aspartate) or placebo every working day during the 2-month training period. The primary source of noise exposure were firearms: 420 shots per person, mean peak level 164 dB(A). The recruits used ear plugs with a mean insertion loss of 25 dB. In both groups Mg-concentration in serum and in erythrocytes increased with time. Lymphocyte Mg increased in the Mg group only. In the placebo group the percentages of ears with PTS > 25 dB at 4 kHz/6 kHz and/or 8 kHz after exposure to firearm noise were twice as high as in the Mg group.