Endothelin-1 (ET-1) is a 21-amino acid peptide produced by vascular endothelial cells that acts as a potent constrictor of hepatic sinusoids. Hepatic binding of tracer ¹²⁵I-labeled ET-1 was investigated in anesthetized dogs with the multiple-indicator dilution technique with simultaneous measurements of unlabeled immunoreactive ET-1 plasma levels. Despite 80% binding to albumin, tracer ¹²⁵I-ET-1 was avidly extracted by the liver, with only 15 ± 6% of the peptide surviving passage through the organ. Exchange of ET-1 between plasma and binding sites, probably located on the surface of liver cells, was quantitatively described by a barrier-limited, space-distributed variable transit time model. Reversible and irreversible parallel binding sites were found. Reversible and irreversible plasma clearances of unbound ¹²⁵I-ET-1 were 0.084 ± 0.033 ml · s¹ · g liver¹ and 0.17 ± 0.09 ml · s¹ · g liver¹, respectively, and the dissociation rate constant for reversible binding was 0.24 ± 0.12 s¹. The specific ET_A receptor antagonist BMS-182874 did not modify binding to either site. The nonspecific ET_A/ET_B antagonist LU-224332 dose-dependently reduced irreversible binding only. ET-1 levels in the hepatic vein were significantly lower than in the portal vein but were not different from those in the hepatic artery. The ratio between hepatic vein and portal vein levels (0.64 ± 0.31) was considerably higher than survival fractions, suggesting a substantial simultaneous release of newly synthesized or stored ET-1 by the liver. These results demonstrate both substantial clearance and production of ET-1 by the intact liver. Hepatic ET-1 clearance is mediated by the ET_B receptor, with the presence of reversible, nonspecific ET-1 binding at the liver surface