Tracheal insufflation of chloramphenicol acetyltransferase (CAT) expression plasmid (pBL-CAT) DNA without any gene delivery system results in transfection of rat lungs (Am J Physiol. 1995;268:L1052-L1056). This study investigated the pulmonary response to tracheal insufflation of plasmid DNA and the immunohistochemical localization of transgene expression in the lungs. Insufflation of 300 micrograms pBL-CAT DNA resulted in lung transfection without causing pulmonary injury as judged from the protein content of alveolar lavage fluid, pleural effusion, and lung ultrastructure. There was, however, an acute alveolar inflammatory response at 6 h after insufflation due to contaminating endotoxin present in the plasmid DNA preparation. Reducing the amount of endotoxin from 0.022 to 0.0015 microgram per 300 micrograms pBL-CAT completely abolished the acute alveolar inflammatory response without affecting the lung transfecting efficiency of pBL-CAT. Immunohistochemistry revealed that insufflation of pBL-CAT transfected predominantly small airway epithelial cells.