The incidence of hypomagnesaemic tetany in cattle and sheep can be reduced considerably by increasing the dietary intake of magnesium (Allcroft, R., 1954; Allcroft, W. M., 1947; Bartlett et al., 1954; Stewart, 1954) and the commonest method of supplementing the ration is to provide a mineral mixture containing magnesium oxide. While the feeding of these mineral supplements to animals wintered indoors is relatively straightforward, ensuring that animals grazing on hill and marginal land will obtain a regular supply of the mineral raises a problem. Where animals are receiving silage in addition to natural grazings during the winter, the inclusion of a magnesium compound in the grass at the time of ensiling is one method of increasing the magnesium content of the diet. The oxide and carbonate of magnesium are probably the safest compounds for feeding to stock, but unfortunately these are likely to have an adverse effect on the preservation, resulting in inferior silage. Experiments carried out in small silo units at Boghall Farm proved this to be the case (E.E.S.C.A. Ann. Rep. 1955). Magnesium sulphate is likely to have little effect upon the preservation of silage, and although this compound is not ideally suitable as a magnesium supplement owing to its laxative properties, it was decided to investigate the value of using it in restricted quantity and intimately mixed with the silage, as a suitable means of preventing hypo-magnesaemia when fed during the winter to a herd of 18 outwintered Galloway cows.