The influence of sodium ions on gastric absorption of calcium in ruminants was studied with regard to the accompanying anions. Bags made of the surviving mucous membrane of different compartments of bovine stomach were used for this purpose. The epithelial side of the mucous membrane always faced towards the inside of the bags. The bags were filled with a combined solution of a calcium salt (chloride, carbonate or phosphate) and sodium chloride. The concentration of calcium was always the same (10 mg/100 ml) while the concentration of sodium varied, amounting to 102.5, 205 and 1025 mg/100 ml. The external face of the bags was placed in Tyrode's solution. In general, calcium absorption became more and more intensive when the concentration of sodium in the contents increased, regardless of the solubility and dissociability of the calcium salt used. When chloride was the accompanying anion, this effect appeared in the rumen and omasum at a physiological concentration of sodium (205 mg/100 ml), and in the abomasum it appeared only at a concentration which widely exceeded the limits of the normal (1025 mg/100 ml). When either carbonate or phosphate were used for this purpose, this effect generally appeared at the highest concentration of sodium, and only in the abomasum, with phosphate as the accompanying anion, at a concentration of 205 mg/100 ml. On the basis of these results, it was concluded that the presence of sodium ions in the contents stimulates gastric absorption of calcium, but the concentration of sodium at which this stimulative effect appears depends, at least to a certain degree, on the accompanying anion of calcium. The possibility of improving the absorption of calcium in the digestive tract of ruminants is discussed in view of the results obtained.