Basic principles of in vivo kinetics are reviewed as they apply to the digestive process in ruminants. Emphasis is on application and limitations of techniques for measuring ruminal turnover of undigested feed residues. This turnover can best be measured by using feed particles whose organic constituent(s) contain uniform concentrations of an isotope. Alternatively, rare earths remain attached to feed residues throughout the digestive tract and can also be used. Appropriate application of mathematical methods must consider ingestaflow as a multicompartment process.