In rumen acidosis, induced by infusion of saccharose solution and solutions of different volatile fatty acids and lactic acid into the rumen, and during induced disturbances of acid-base equilibrium in the arterial blood the motility of the dorsal sac of the rumen, the pH of rumen content and the indices of acid-base equilibrium in the arterial blood were investigated. The pH and the indices were determined by the micromethod of Astrup with an Acid-Base-Cart ABC-1 unit. During saccharose-induced acidosis of the rumen the pH of its content was decreased and its motility was strongly inhibited. Acidification of rumen content corresponded to the dissociation constant of a given acid. The motility of the rumen was inhibited most strongly by butyric acid, followed with regard of this effect by acetic acid, propionic acid and lactic acid. It was found that hydrogen ions as well as anions and non-dissociated forms of acids produced in the rumen were responsible for inhibition of this motility and that the rise in the concentration of hydrogen ions in the blood had no inhibitory effect on the motility of the rumen.