The data presented in this manuscript describes the binding characteristics of [3H]SB 209670, a potent nonpeptide tritium-labeled endothelin (ET) receptor antagonist. The binding of this antagonist to cloned human ETA and ETB receptors was specific, saturable and of high affinity. The apparent dissociation constants were 0.20 and 1.0 nM for ETA and ETB receptors, respectively. The maximum binding was 4.7 and 22.5 pmol/mg protein for ETA and ETB receptors, respectively. Unlike ET-1, the binding of [3H]SB 209670 was reversible. The half-times (T1/2) for dissociation of this ligand from ETA and ETB receptors were approximately 60 and 10 min, respectively. Competition binding studies using [3H]SB 209670 and unlabeled agonists ET-1, ET-3 and S6c indicated that these agonists displayed similar affinities for human ETB receptors, whereas with ETA receptors, ET-1 was approximately 50-fold and 1500-fold more potent than ET-3 and S6c, respectively. Of the peptide antagonists tested, BQ123 (ETA-selective peptide antagonist), displayed Ki values of 40 and > 2300 nM for ETA and ETB, whereas RES701 (ETB-selective antagonist) displayed Ki values of > 1600 and 81 nM for ETA and ETB receptors, respectively. The nonselective peptide antagonist, PD 142893, was approximately 2-fold more potent for ETA compared with ETB receptors. Similar observations were made with nonselective nonpeptide antagonists, Bosentan, (+/-) SB 209670, SB 209670, and (-) SB 209670. All these compounds were 2 to 10 times more potent for ETA than ETB receptors.