According to Annex 50 of the German Effluent Treatment Law, dental aspirator units must be fitted with an amalgam separator, and a minimum separation efficiency of 95 % must be achieved. Dental aspirators are frequently disinfected at the end of each working day and before the week-end, although this is not essential on purely hygienic or environmental health grounds ; it is nevertheless a common practice and afterwards a certain amount of finely dispersed dental amalgam remains in contact with the disinfectant solution in the separator for a considerable time. Due to oxidation of the amalgam, mercury is remobilised, the extent of remobilisation being dependent, in the case of aged amalgam, on the oxidation potential of the disinfectant. The maximum mercury concentration permitted under the regulations governing indirect discharges in Baden-Württemberg, namely 5 μg/l, was substantially exceeded in some of the rinsing waters in our investigations due to remobilisation. Nevertheless the maximum permissible load (0.1 Hg/h) was not attained even with strongly oxidising disinfectants. In the case of freshly prepared amalgam the amount of mercury remobilised was independent of the nature of the disinfectant. For aged amalgam the quantity of mercury remobilised increases with the oxidation potential of the disinfectant concerned. It is possible to reduce the level of mercury and AOX emissions resulting from the disinfection of dental aspirator units quite substantially at no great cost by choise of suitable cleaning agents and disinfectants. It remains to be shown whether cleaning without disinfection, or the use of disinfectants not containing oxidising agents, can achieve the required level of hygiene.