Calcined magnesite is given to ruminant animals as a preventive measure against hypomagnesaemic tetany resulting from magnesium deficiency. The major objectives of this thesis were to investigate the dietary availability of the four major calcined magnesites available on the UK feed market and to determine the consistency of these results by investigating four different samples of each over a 2 year period using a standard technique in comparison with a standard magnesium hydroxide. Using the bioavailability results obtained it was hoped to determine a quick in vitro test to correlate with apparent availability.