The ruminant has to be able to regulate potassium metabolism over an extremely wide range of intake. When related to body weight, dietary potassium intake of ruminants frequently exceeds that of humans by a factor of over ten, and experimental situations suggest that the toxic threshold for ingestion of potassium is considerably higher. While a number of studies have addressed the outstanding ability of the ruminant kidney to extrude potassium, the role of the rumen in maintaining potassium homeostasis in sheep and cows by serving as a reservoir for the redistribution of potassium has received considerably less attention. Based on current and past studies at the level of the cell, the tissue and the animal, we present a model for the regulation of ruminal transport that may help in the understanding of the variable responses of the rumen epithelium to dietary potassium intake, and the role of magnesium in mediating these responses.