These studies were designed to calculate the fractional disappearance rate (FDR) and splanchnic extraction of insulin in response to an exogenous (intraperitoneal) input of insulin. A double-tracer technique using insulin tritiated on both the A1 and B1 positions was introduced for the measurement of hepatic extraction. The A1 tracer, not previously characterized in vivo, was compared in terms of its kinetics with H3-B1-insulin and unlabeled insulin. The metabolic clearance rates (MCR) of the three insulins were identical, as were the decay curves of the two tracers. To measure splanchnic insulin extraction, one tracer was infused systemically to evaluate the FDR of insulin, and the second was infused into the splanchnic circulation (superior mesenteric artery) and its peripheral appearance was calculated. Splanchnic extraction was determined from the difference between this rate of appearance and the rate of infusion of the mesenteric tracer. After intraperitoneal insulin injection, insulin levels increased to peaks of 549 +/- 93 microU/mL (portal vein) and 473 +/- 99 microU/mL (inferior vena cava) and decreased to basal levels over 3 hours. The FDR decreased from 0.295 +/- 0.051 min-1 to 0.125 +/- 0.026 min-1, and splanchnic extraction decreased from 0.534 +/- 0.06 to 0.232 +/- 0.088. The latter returned to near-basal values more rapidly than did the FDR. In conclusion, the kinetics of insulin both in and out of the steady state have been shown to be nonlinear through physiological insulin concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)