Improvements in the management of premature infants have resulted in a dramatic increase in their survival. Previously, nutritional management was not of high priority in very low birth weight (VLBW) infants since most died early in life. As a result, our refinement of the nutritional management of these surviving infants has lagged behind the advancements in cardio-respiratory therapy. VLBW infants are susceptible to hypoglycemia; thus, to provide sufficient calories and to prevent hypoglycemia, glucose is routinely infused at high rates (approximately 60 mumol/kg min, twice that of measured glucose turnover rates in these infants) as part of standard total parenteral nutrition (TPN) which frequently results in hyperglycemia. Like hypoglycemia, hyperglycemia can have significant short and long term adverse impact on their outcomes. The specific focus of the present application is to determine the individual and collective roles of the substrates provided in standard total parenteral nutrition on glucose production from glycogenolysis and gluconeogenesis (GNG) and glucose utilization. These studies will determine: a. Whether VLBW infants receiving TPN who are normoglycemic suppress their glucose production completely and those who are hyperglycemic have incomplete suppression of glucose production and/or reduced glucose utilization; b. When VLBW infants are supplied glucose at a reduced rate (17 mumol/kg min): 1. Do i.v. lipids (IntralipidR) increase GNG and glucose production and if so is this the result of providing energy for GNG through beta-oxidation or providing glycerol as a 3-carbon precursor; 2. Whether glycerol increases GNG and glucose production in a dose dependent fashion in VLBW infants; 3. Whether i.v. amino acid solution (TrophAmineR) and/or the gluconeogenic amino acids, glutamine and alanine, will increase GNG and glucose production; and 4. Whether glucagon will increase GNG and glucose production from endogenous and exogenous substrate sources. The ultimate aim will be to utilize this information in designing alternative mixtures of constituents (substrates and possibly hormones) which will decrease the risk of both hypo- and hyperglycemia in these infants, by utilizing the infants' gluconeogenic pathway, while providing sufficient substrate to sustain normal growth and development.