1. A low magnesium diet is described, consisting of barley straw, sucrose and casein, which induced hypomagnesaemia and one case of clinical tetany in non-pregnant, non-lactating ewes.
2. On the above diet a daily drench of 0·25 g. MgO maintained plasma magnesium levels at 1·60–1·80 mg./100 ml. which was significantly higher than the plasma magnesium level of 1·09–1·20 mg./100 ml. for the unsupplemented group.
3. An experimental formulation of a magnesium heavy pellet is described and its use as a method of supplementation was investigated. On the above diet it was found that two or four heavy pellets also maintained plasma magnesium values at 1·601–80 mg./100 ml. which was significantly higher (P = 0·01) than the level of the unsupplemented group.
4. Five sheep out of the forty ewes used in this experiment were found to have consistently lower plasma magnesium values than the remaining thirty-five.
5. A preliminary trial of the use of heavy pellets as a magnesium supplement in non-pregnant, non-lactating ewes on winter grazing is described. The results of the trial under these conditions were encouraging.