The accuracy with which the secretion rate of insulin can be calculated from peripheral concentrations of C-peptide was investigated in conscious mongrel dogs. Biosynthetic human C-peptide and insulin were infused intraportally and their concentrations measured in the femoral artery. During steady-state infusions of C-peptide, the peripheral concentration changed in proportion to the infusion rate and the metabolic clearance rate (5.2 +/- 0.3 ml/kg/min) remained constant over a wide range of plasma concentrations. Application of a two-compartment mathematical model, in which the model parameters were estimated from analysis of C-peptide decay curves after intravenous bolus injections, allowed the intraportal infusion rate of C-peptide to be derived from peripheral C-peptide concentrations, even under non-steady-state conditions. Estimates of the intraportal infusion rate based on this model were 102.4 +/- 2.6% of the actual infusion rate as it was increasing and 102.3 +/- 5.5% of this rate as it was falling. The peripheral C-peptide: insulin molar ratio was influenced by the rate at which equimolar intraportal infusions of C-peptide and insulin were changed. The baseline C-peptide: insulin molar ratio (4.1 +/- 0.9) increased to peak values of 8.2 +/- 0.6, 10.3 +/- 2.0, and 14.9 +/- 1.3 when the infusion rate was increased and then decreased rapidly. Peak values of only 5.7 +/- 1.2 were found if the intraportal infusion rate was changed slowly.(ABSTRACT TRUNCATED AT 250 WORDS)