This chapter examined the applicability of the splanchnic/hepatic balance technique to the study of glucose metabolism in humans and in dogs. When combined with radioisotope turnover methodology, the hepatic/portal venous catheterization technique provides a powerful tool to investigate splanchnic/hepatic carbohydrate metabolism. It should be emphasized that the same principles described on the preceding pages can be applied to study amino acids (Gelfand et al, 1986), potassium (DeFronzo et al, 1980) and insulin (Ferrannini et al, 1983), as well as a variety of other metabolites and hormones.