Recurrent Airway Obstruction (RAO) is a chronic, worldwide recognized diseases ot horses. Despite the well known clinical presentation and the ease of diagnosis, the exact etiology and pathogenesis are still unclear which makes it difficult prevention and treatment. Recurrent obstruction of airway is caused by mucus production, bronchospasm and bronchial edema and the most important clinical signs are chronic cough and expiratory effort. The ET-1 is the most potent vasoconstrictor molecule up to now identified, produced by several cells as endothelial cells, smooth muscle cells and epithelial cells of airways. The ET-1 is a potent proinflammatory, secretagogues and bronchoconstrictors mediator. ET-1 can cause bronchial contraction on bronchial rings of horses in vitro, especially in horse with respiratory disease.