To determine the impact of biosynthetic human insulin (BHI) on endogenous insulin release, splanchnic output and arterial concentrations of C-peptide were measured in eight healthy men after intravenous administration of 0, 0.5, 1.25, U BHI . m-2 . h-1 for 70 min each. Euglycemia was maintained by a variable glucose infusion. Arterial levels of serum insulin were 48 +/- 6 pmol/liter before and 135 +/- 12, 265 +/- 18, and 593 +/- 47 pmol/liter after BHI infusion. Splanchnic C-peptide output was reduced by BHI infusion from 88 +/- 10 pmol/min before to 50 +/- 9, 28 +/- 10, and 18 +/- 16 pmol/min (P less than 0.0025). Simultaneously, arterial concentrations of C-peptide fell from 539 +/- 54 pmol/liter by 29 and 43% when 1.25 and 2.5 U . m-2 . h-1 of BHI were administered. Hepatic insulin uptake was directly related with BHI infusion rate (r = 0.88) and rose during BHI administration from a basal value of 58 +/- 7 to an uptake of 265 +/- 31 pmol/min when 2.5 U . m-2 . h-1 were infused (P less than 0.0005). Basal hepatic insulin clearance was 4.75 +/- 0.60 ml . kg-1 . min-1 and remained unchanged after BHI infusion as did hepatic fractional extraction of insulin, which was 61 +/- 4% in the basal state. Metabolic clearance rate of immunoreactive insulin (MCRi) was dose-dependently reduced by BHI infusion, whereas the relative share of hepatic insulin clearance in total MCRi rose simultaneously (P less than 0.01). We conclude that feedback inhibition of endogenous insulin release may play an important role in vivo. Furthermore, it appears that nonhepatic insulin degradation is a saturable phenomenon as total MCRi fell in the presence of its unchanged hepatic clearance rate after the infusion of large amounts of BHI.