Technical Abstract: The prevalence of overweight and obese children has doubled and the incidence of type II diabetes in children (0-19 years) has increased fourfold over the past several decades. As a result we can anticipate an increased number of metabolic studies in children. There are few data on parameters of glucose metabolism in normal children, and virtually none relating to their reproducibility. The aim of this study was 1) to provide new data on energy expenditure, glucose, lipid and protein metabolism in non-obese, healthy children and adolescents; 2) evaluate their reproducibility; and 3) based on these data to perform power calculations for metabolic studies. Eight non-obese subjects (8-16 years) were studied on two occasions, preceded by 7 d of a diet with identical energy content and macronutrient distribution. Gluconeogenesis, measured by deuterium oxide, accounted for 50% of glucose production. Insulin sensitivity, measured by the labeled minimal model, averaged 4.9 x 10**-4 min-1 /uU mL- Glucose appearance rate was significantly higher (p<0.01) in the children than in the adolescents. Further, we demonstrate that for energy intake and expenditure, plasma concentrations of glucose and C-peptide, and rates of appearance of glucose and leucine, a 10% difference can be detected in less than 5 subjects with a power of 80% and a type II error of 5%. Insulin concentration, gluconeogenesis, insulin secretory indices, insulin sensitivity and glucose effectiveness were more variable, but with the above power a difference of 25% could be detected in 7-11 subjects using a paired study design.