To investigate the influence of alterations in protein synthetic and degradative rates to the regulation of muscle mass, a variety of laboratory techniques have been developed in order to estimate the rates of total protein and individual contractile protein turnover in the intact experimental animal. These techniques are based on well-established methods of compartmental analysis, and rely on the intravenous administration and biosynthetic incorporation of radiolabeled amino acids into newly synthesized muscle proteins. In this review, the two most widely used procedures (the constant and flooding infusion methods) are examined with respect to the major assumptions and pitfalls in the two procedures. The theoretical and practical limitations of these biosynthetic labeling techniques are critically analyzed with the aim of providing a clear rationale for the application of these techniques to the future study of skeletal and cardiac muscle growth and atrophy in vivo.