Within a normal range for blood magnesium in cows, 1.55-3.0 mg of Mg per 100 ml of serum or plasma, statistically significant differences between individuals were detected. 2. Reference was made to studies of the renal handling of magnesium and to demonstrations in the literature of large individual variations in the ability of cows to utilize magnesium from the diet. It is logical to assume that such variations will contribute to the creation of individual differences in plasma levels of the element. The point was emphasized that to moderate differences in plasma concentration can correspond large quantitative differences in magnesium metabolism. 3. A positive correlation was demonstrated in one herd between earlier individual plasma magnesium levels within the normal range and decreased levels after one week on a tetany-producing pasture. 4. Gows with initial plasma magnesium values in the lower part of the range of a group of animals appeared specially susceptible to disease when subsequently the whole group was subjected to hypo- magnesaemia-producing conditions. This rule was detected within the limited material from the mentioned herd with tetany and also in materials from the literature. 5. The possibility that certain individuals within herds can be pointed out as specially susceptible to hypomagnesaemic disturbances before the beginning of a tetany season may be of practical interest.
