While realizing the difficulties with the various methods used to study hormonal control of protein metabolism, there appear to be clear effects of both rapid-acting and slower-acting hormones. Moreover, some of these hormones affect protein metabolism in a dose dependent manner. Insulin and IGF-I appear to have differing effects at lower doses, with insulin primarily inhibiting protein degradation and IGF-I stimulating protein synthesis. At higher doses, infusions of insulin and IGF-I both seem to inhibit protein degradation and stimulate protein synthesis. Epinephrine primarily inhibits protein degradation whereas growth hormone primarily increases protein synthesis. Infusion of amino acids themselves can also increase protein synthesis. Thyroid hormone excess increases protein synthesis and protein degradation, with the latter effect predominating. Sex steroids appear to increase protein synthesis. To date, most interventions studying the metabolic effects of these hormones on protein metabolism have involved varying the concentration of one hormone at a time. In the complex milieu of many pathologic states (e.g. sepsis, renal failure or even the transition from fasting to feeding) multiple hormones change simultaneously. How interactions among these factors determine the overall response of body and muscle protein remains to be defined.