Selenium (Se) deficiency has been recognised for over 30 years as a factor which limits production in grazing livestock. Diseases associated with selenium deficiency in New Zealand include nutritional myodegeneration (white muscle disease) of lambs,calves and goats, infertility of ewes and ill-thrift of sheep and cattle. Although white muscle disease is the most recognisable manifestation of deficiency, sub-clinical deficiencies are more common. Such deficiencies result in decreased growth rate, milk production and lambing percentage (Grace, 1994). The wide distribution of soils that are deficient in selenium (Watkinson, 1983) means that Se deficiency and its amelioration has considerable economic importance to New Zealand. Also of importance is lost revenue due to inappropriate supplementation leading to unnecessary expense and, on occasion, losses due to toxicity. There is a need to avoid excessive treatment to minimise the risk of selenium concentrations in ruminant liver and kidney tissue exceeding the acceptable concentration for human consumption. The development of reference ranges for the prevention of ill-thrift and myodegeneration in young stock has been the primary aim of selenium research in New Zealand to date. This has left many other aspects of selenium and its role in grazing ruminants to be elucidated. For example, there are few data on which to base reference ranges for adult dairy cattle. This lack of information was an important impetus for the studies presented in this thesis.