The interior of the rumen in cattle and sheep is normally maintained at a potential of about -40 mV relative to the blood. This potential depends primarily on the occurrence of an active transport of sodium from rumen to blood, since the potential, short-circuit current and the net sodium flux are simultaneously abolished by anoxia, ouabain and removal of sodium from the bathing solutions. There is an appreciable net flux of potassium from blood to rumen. There is also a substantial active transport of chloride in the same direction as sodium and it can be reduced by treatment with acetazolamide without affecting the potential or the sodium system. Nevertheless, sodium transport is reduced by the removal of chloride ions. Omasum epithelium is similar to rumen epithelium. However, the chloride pump appears to work in both directions in this tissue. Short-circuited omasum epithelium can also transport magnesium from omasum to blood.