The classical investigations of Borsook and Dubnoff showed that creatine is synthesized in the liver (l-3). From these studies carried out in vitro on homogenates and liver slices, no definitive conclusions may be drawn concerning the rate at which creatine is synthesized in viwo. Furthermore, recent studies have indicated that creatine may also be synthesized in organs other than the liver (4,5). In the past few years, interest in creatine metabolism has been renewed, particularly since it appears that creatine synthesis may be regulated by the creatine content in the diet (6-9). Although the experimental results suggest that creatine synthesis may be controlled indirectly by the level of transamidinase in the kidney (7, 8, lo), the effect of creatine intake on the hepatic synthesis of creatine from glycocyamine has not been measured directly. In the present investigation, the rate at which creatine is synthesized from glycocyamine in the isolated perfused liver has been determined under two experimental conditions: (u) in the presence of precursors at levels normally present in blood and (b) in the presence of high concentrations of precursors added to the perfusion. The rate at which creatine is synthesized in the isolated perfused liver taken from rats fed a diet containing 2% creatine for 5 days was also determined. Finally, extrahepatic creatine synthesis was studied in the surviving eviscerated rat after surgical removal of all abdominal viscera.