On the rat acute lung injury model produced by intraperitoneal cavity injection of zymosan showed that the respiratory rate became slow at first time (1st day), subsequently became rapid. Pulmonary histology showed that there was an accumulation of neutrophils in pulmonary microvessels, neutrophils infiltration in interstitial tissue, interetital and olveolar edema, hyaline membrane, atelectasis. Positive expression of ET-1 in experimental rats was stronger than than contral rats. Image analysis showed that OD value was elevated, especially at the fist day. The quantity of ET-1 was corresponding to the degree of pathologic change of acute lung injury. It suggests that the ET-1 might play an important role in the mechanism of acute lung injury.