Abstract: Mercury (Hg) is one of the most toxic heavy metals. From a biological perspective it has no redeeming virtue, for, unlike a number of other heavy metals, it is not known to perform any essential biochemical function (Bowen, 1966). Traces of Hg are ubiquitous in soils, natural waters, sediments, organisms and air (Jonasson and Boyle, 1972), and anomalously high Hg concentrations occur in many ecosystems owing to Hg pollution (a serious, widespread problem), natural Hg enrichment in certain rocks, distinctive properties of Hg such as its tendency to form highly stable complexes and compounds (including species that are easily taken up by organisms but not readily excreted), natural processes (e.g. methylation) which enhance the bioavailability of Hg, increased bioavailability of Hg due to environmental changes caused by human activities, and efficient accumulation of Hg by organisms and certain natural materials, such as soil organic matter and fine-grained sediments. Moreover, Hg is a relatively volatile element, and this accounts, in large part, for its wide distribution.