1. A procedure is described for using pivalic acid as an inert reference substance in determination of changes in concentrations of volatile fatty acids (VFA).
2. Pivalic acid in concentrations of up to 80 mmol/l had no effect on production of methane or VFA by rumen contents.
3. Pivalic acid was inert during incubation with rumen contents from sheep given different diets and with samples taken at different times with respect to feeding.