The endothelins (ETs) are potent vasoconstrictor peptides that exert biological effects via specific receptors, including the ETA subtype (selective for ET-1) which is predominant in vascular smooth muscle. ET has been implicated in the pathophysiology of various diseases, but proof of its causative role has been hindered by the lack of orally active antagonists with long half-lives. While structurally diverse ET antagonists (e.g., cyclic pentapeptides) have been reported, orally active ones are needed as optimal tools and potential therapeutic agents. In this study, we discovered benzenesulfonamide ETA receptor antagonists through random screening in an ETA binding assay and conducted structure-activity studies. Optimization led to the naphthalenesulfonamide 11 (BMS-182874), a potent, orally active, and highly ETA-selective antagonist. Compound 11 showed an ETA binding IC50 of 150 nM (Ki = 55 nM), inhibited ET-1-induced intracellular Ca2+ increase in vsm-A10 cells with an IC50 of 570 ± 70 nM, and had a KB value of 520 nM in rabbit carotid artery rings. It exhibited >3600-fold selectivity for ETA over the ETB receptor (Ki >200 pM for ETB). In one-kidney DOCA-salt hypertensive rats, a single oral dose of 11 (100 μmol/kg) reduced mean arterial pressure by 25% within 1 hour, with a 12% reduction still observed 24 hours post-dosing, demonstrating oral activity and long duration of action. Compared to BQ-123, which has an extremely short in vivo half-life and showed a poorer hypotensive effect in a similar model, 11 exhibited a more pronounced and longer-lasting antihypertensive effect. In summary, optimization of benzenesulfonamide ligands led to N-isoxazolyl-1-naphthalenesulfonamide ligands, and the 5-dimethylamino analogue 11 is an orally active, non-peptide, highly ETA-selective receptor antagonist that is proving useful for elucidating the role of ET in animal models of human disease.