At the present time, the economy of New Zealand is largely dependent upon the health end well-being of the ruminant animal. The national loss due to primary water and electrolyte disturbances, and to those secondary to other diseases, is of considerable importance. Deficiencies in our knowledge of water and electrolyte metabolism in the ruminant have become apparent, even of the principal cations sodium and potassium. The specialized form of nutrition in the ruminant has entailed some changes in the water and electrolyte economy. In adapting to a diet of plant material rich in cellulose, they have developed a large forestomach, the reticulo-rumen, where a symbiotic population of bacteria and protozoa is maintained and exploited. Microbial fermentation breaks down plant cellulose and converts carbohydrate to volatile fatty acids, principally acetic, propionic and butyric acids, which are rapidly absorbed by the host for use as an energy source. Microbial protein and certain vitamins are also made available further down the gastro-intestinal tract. The development of the reticulo-rumen has resulted in an increase in the content and daily turnover of gut water and electrolytes. A major source of this content of the reticulo-rumen liquor, which provides a well-buffered medium for the microbes, is the copious salivary flow. The rumen of the sheep may contain 500-800 m-equiv of Na+, approximately half that in the extracellular fluid. The daily digestive cycle of salivary secretion and later reabsorption may involve double this amount of Na+.