The rate of glucose production by liver is a function of the supply of precursor substrate released by the peripheral tissues. The most important amino acid precursor released by muscle and incorporated into glucose in liver is alanine. Therefore, the synthesis and release of this amino acid by muscle may ultimately limit the rate of gluconeogenesis. The origin of the carbon and nitrogen moieties of alanine are believed to be coupled to muscle cell glycolysis and the transamination of the branched chain amino acids, respectively. A "glucose-alanine cycle" is thus formed between muscle and liver and provides for both a conservation of carbohydrate similar to the Cori-lactate cycle and a tpansport of nitrogen in a non-toxic form between muscle and liver. Such a cycle, however, does not provide a mechanism for the conversion of muscle protein carbon into carbohydrate, a process believed to occur in the fasting and uncontrolled diabetic state. Although no direct evidence is available, other investigators have suggested amino acid may feed carbon into the pyruvate pool so that the carbon skeleton of alanine ultimately arises from amino acids released via proteolysis. We have recently documented the viability of an established myogenic cell line of rat skeletal muscle cells in tissue culture as a suitable preparation for the study of muscle amino acid metabolism at the cellular level. With this preparation we plan to correlate glucose, pyruvate, and amino acid utilization with alanine production in order to obtain direct evidence regarding the origin of the carbon skeleton of alanine under the influence of various nutritional and hormonal modulations.