Concentrations of magnesium in plasma, the ionic concentration in rumen digesta supernatant fractions, Mg balance and electropotential differences were measured in three ewes which were fed on grass and supplemented with potassium and Mg by intraruminal infusion. 2. Mean plasma Mg concentrations were unaltered by combined K and Mg treatments, but fell (P less than 0.001) when K alone was infused. 3. The mean concentrations, in rumen digesta, of sodium and K varied reciprocally (P less than 0.001) with each other when K was infused, but were unaffected by Mg infusion. The mean Mg concentrations in rumen digesta fell (P less than 0.01) with K infusion but rose (P less than 0.001) with Mg infusion. 4. Absorption and excretion of Mg rose (P less than 0.001) when Mg intake was increased but was unaffected by K intake.
