We have recently demonstrated in the guinea pig that preventive dietary magnesium supplement can significantly reduce impulse noise induced hearing loss by on average 18 dB. The purpose of the present study was to examine whether magnesium might also have a therapeutic effect on noise trauma. Anesthetized guinea pigs were exposed to an impulse noise series (1/s) of L(peak) 167 dB (L(eq,ls) 127 dB) for 38 min. The permanent hearing threshold shift (PTS) was determined one week post-exposure, using auditory brainstem response audiometry at a frequency range of 0.5-32 kHz. The total magnesium concentrations of perilymph (PL), cerebrospinal fluid and blood were analysed by atomic absorption spectrometry at different times of treatment. In a first set of experiments, animals on a low initial magnesium status were injected either of 4 different dose levels of magnesium (1.14-3.42 mmol MgSO4 s.c./kg per day) for 3 days or saline as a placebo. The treatment was started immediately after the exposure. The magnesium groups received drinking water with an additive of 39 mmol MgCl2/l for one week and the placebo group tap water (0.43 mmol Mg/l) alone. A dose level of 2.85 mmol Mg has proved to be most effective and reduced the PTS by 13-20 dB compared to the placebo group. The magnesium concentrations increased to above 4 mmol/l in serum and to 1.2 mmol/l in PL during the first 3 days of this treatment. In a second set of experiments, we tested the dependence of the therapeutic efficacy on the post-exposure time of onset of the optimal treatment (1 min, 2 h and 4 h), using guinea pigs on normal initial magnesium status. In the 1 min-group, the reduction of hearing loss was similar to that found in the first series. The therapeutic effect decreases with the length of time elapsed between the end of exposure and the beginning of treatment. In a few animals, hair cell stereocilia were examined using scanning electron microscopy. The results also revealed a magnesium related reduced susceptibility of hair cell stereocilia to impulse noise exposure.