The rates of production of acetic, propionic, and butyric acids in the normal rumen of a sheep have been measured from the decline in specific activity of these acids in the rumen after introduction of their sodium salts, labelled with 14C. Polyethylene glycol was introduced into the rumen to permit measurement of the rumen volume. The rates of production were: acetic acid 99 m-moles/hr, propionic 44 m-moles/hr, and butyric 21 m-moles/hr.