The placental alkaline phosphatase (PLAP) is a marker enzyme of alveolar type I cells. To evaluate the damages of alveolar type I cells and its relations with the development of acute lung injury caused by smoke inhalation, the present study was designed to observe the dynamic changes in PLAP contents in plasma and bronchoalveolar lavage fluid (BALF). Following the induction of smoke inhalation injury in rat, the arterial blood gas levels, lung water volume, total protein and albumin contents in BALF, and PLAP contents in plasma and BALF were determined respectively in normal control and injured animals at 2, 6, 12 and 24 h after injury. The pathomorphology of lung tissues was also observed. It was found that after smoke inhalation, animals showed acute respiratory failure and serious pulmonary edema. The total protein and albumin levels in BALF increased markedly. Both PLAP contents in plasma and BALF also increased dramatically, and there was a significant positive correlation between the changes in the PLAP and the total protein contents in BALF. The pathomorphologically serious structural damage of alveolar type I cells were also found. PLAP may be not only a marker of alveolar type I cells injury, but also interrelated with the permeability increase of alveolar-capillary membrane after smoke inhalation.