Methods that were originally developed for animal studies have now been modified for measuring tissue protein turnover in man with stable isotopes. Of the three techniques which have been discussed, only the arterio-venous difference method has the capacity to measure both the synthesis and degradation. This technique is, however, difficult to perform and has a number of potential sources of error, one of the most important being the assessment of precursor labelling. Constant infusion of a labelled amino acid with measurement of the incorporation into protein of a biopsy is a simpler and more precise technique for measuring synthesis in patients as well as volunteers, but the most appropriate means of assessing the precursor labelling still remains to be worked out. The flooding-dose procedure aims to minimize the difficulty of assessing precursor labelling and there is no evidence that the unphysiological dose of labelled amino acid given influences the synthesis rate which is measured. It is rapid to perform and is very well suited to measurements in patients, in whom a wide range of tissues can be studied. These advances in techniques have been facilitated particularly by improvements in mass spectrometry, which have allowed the use of stable in place of radioactive isotopes, and have enabled measurements of isotopic enrichments to be made in small samples of tissue. These techniques for assessing the dynamics of protein metabolism in individual tissues are now being used to answer nutritionally and clinically important questions in human volunteers and patients.