Recently, we have shown a substantial increase in the endothelin-1 (ET-1) concentration in bronchoalveolar fluid (BALF) during an experimental eosinophilic airway inflammation. Moreover, we observed a significant inhibition of the inflammatory response after treatment with an endothelin receptor antagonist. This indicates that ET-1 may have proinflammatory properties and play a key role in eosinophilic inflammations, such as bronchial asthma. Accordingly, we hypothesized that the synthesis and release of ET-1 precedes the inflammatory response, and that the bronchial epithelium is the site of ET-1 synthesis in the lungs. An eosinophilic airway inflammation was induced by intratracheal Sephadex instillation in rats, and the animals were evaluated after 15 min, 30 min, 1, 2, 3, 6, 12, and 48 h. The ET-1 mRNA synthesis, assessed by Northern and slot blot analyses, was significantly increased 15 min after Sephadex challenge, peaking at 30 min with a 4.7-fold increase, before any signs of inflammation in the BALF could be observed. The increased synthesis was mainly located to the bronchial epithelium and macrophages at sites of inflammation as determined by in situ hybridization. A significant increase in tissue ET-1 was observed 3 h after provocation, and the recruitment of eosinophils followed a substantial release of ET-1 peptide in BALF peaking at 24 h with a 13-fold increase. Therefore, the rapid ET-1 mRNA synthesis and the considerable increase in the level of ET-1 indicate that this peptide plays an important role in the initiation of an eosinophilic airway inflammation.