Stable isotope tracer techniques have been developed to quantify rates of muscle protein synthesis and breakdown in human subjects. These methods were applied to the study of the response to resistance exercise as well as to amino acid intake. The fractional synthetic rate (FSR) of muscle protein is stimulated for as long as 48 h following exercise. However, the anabolic effect of the stimulation of FSR after exercise is blunted by a simultaneous increase in muscle protein breakdown, such that the net balance between synthesis and breakdown remains negative in the fasted state. Elevation of plasma amino acids stimulates muscle protein synthesis. The extent of the stimulation is dependent on the dose, the profile of amino acids given, the pattern of ingestion (bolus vs. constant intake), the age of the subject, and the hormonal profile. Importantly, there is an interactive effect between resistance exercise and amino acids, such that the net anabolic response to amino acids following exercise is greater than the sum of the amino acid effects and the exercise effects alone.