Feline allergic asthma is one of the most common and serious diseases affecting pet cats. Endothelin-1 (ET-1) is a key mediator of asthma through its pro-inflammatory, pro-fibrotic, bronchoconstrictive, and vasoconstrictive properties. Given the high morbidity and mortality associated with feline asthma, a drug that prevents the deleterious sequelae of ET-1 would be clinically valuable in feline patients. The anti-endothelin effects of bosentan, a dual endothelin receptor antagonist (ETRA), and the only commercially available dual ETRA, are well established in many species and have been confirmed experimentally in cats. Bosentan therapy has been shown to dramatically improve outcome in pulmonary disease complicated by bronchoconstriction. By understanding the pharmacokinetics of bosentan, this study will be an important first step in evaluating this drug as a future therapeutic choice for cats with asthma and potentially other diseases mediated by the inflammatory effects of ET-1.