The mortality of acute renal failure (ARF) remains high because most illnesses associated with ARF also cause loss of muscle mass. The mechanisms causing loss of muscle mass are inhibition of protein synthesis and/or acceleration of protein degradation. Identifying which of these abnormalities is present requires understanding the principles of amino acid and protein turnover. There is evidence that ARF by itself impairs protein synthesis and stimulates protein degradation. The mechanisms causing these defects include abnormal metabolic responses to hormones and stimulation of the ATP-ubiquitin-proteasome-dependent pathway. Understanding these mechanisms could lead to rational therapies.