A review of studies concerned with the metabolism of human C-peptide showed that the kidney is the major organ in C-peptide removal. The turnover rate of C-peptide was similar to that of proinsulin but considerably slower than that of insulin. An analysis of corresponding C-peptide and insulin concentrations in normal subjects after the administration of oral glucose or intravenous glucagon was used to define the relationships between the two peptides. These results show that different peripheral vein concentrations of insulin may correspond to identical C-peptide concentrations, depending on sampling conditions.