SUMMARYThe net absorption of magnesium from the temporarily isolated rumen of sheep was measured at different plasma magnesium concentrations. In the first series of experiments, the plasma magnesium concentration was raised from 0-89 + 0-11 to 1-97 + 027 mmol 1-1 by intravenous infusion of MgCl2 immediately before measurement of magnesium absorption. In a second series absorption of magnesium was studied over 20 h after the beginning of a continuous intravenous infusion of magnesium, which led to hypermagnesaemic levels of 2-78 + 0 59 mmol 1-1. In the third series sheep were fed a magnesium-deficient diet which caused a significant decrease of plasma magnesium concentration from 0-96 + 0-06 to 0 50 + 0-06 mmol 1-1. Neither hyper-nor hypomagnesaemia changed net absorption of magnesium from the rumen of sheep. It is concluded that the active ttansport of magnesium from the rumen was not influenced by the plasma magnesium concentration and that any passive and paracellular movement of magnesium through the rumen wall must have been negligibly small under the conditions of these experiments.