Laboratory experiments have indicated that selenium acts as a powerful antagonist to mercury intoxication. The literature is reviewed and from this it is concluded that mercury and selenium react in various ways. 1) The mercuric ion binds to selenium to form a biologically inert complex leading to increased body burden of both elements. This reaction seems to take place only when a threshold of mercury exposure is exceeded. 2) Selenium influences the oxidation rate of elemental mercury and as such the distribution pattern. This reaction is influenced by the nature of the antioxidative system. In species with low glutathione peroxidase activity, mercury oxidation seems decreased which can lead to an increased brain uptake. In this process there is no threshold. 3) Selenium can, as part of the antioxidative defence system, counteract mercury induced lipid peroxidation. Other antioxidants e.g. vitamin E might be just as effective. Based upon the literature it is hypothesised that to man selenium is of no benefit in cases of exposure to mercury either as mercuric mercury or as vapours. The only protection against mercury will still be a low exposure level.