Objective(s): Gastroesophageal Reflux Disease (GERD) is one of the most common digestive disorders that frequently lead to pulmonary complications due to gastric fluid aspiration. In the present experimental study, chronic aspiration of gastric fluid, its components and bile salts in rat lung was performed to find out the main factor(s) causing pulmonary complications of gastric fluid aspiration. Materials and Methods: Forty eight male rats weighted 250-300 g were selected in six groups. After anesthesia and tracheal cannulation, the animals received 0.5 ml/kg normal saline, 0.5 ml/kg of whole gastric fluid, 0.5 ml/kg pepsin (2.5 µg/ml), 0.5 ml/kg hydrochloric acid (pH=1.5) or 0.5 ml/kg bile salts (2.5 µg/ml) by injection into their trachea and lungs. In sham group nothing was injected. Results: Parenchymal and airways inflammation and fibrosis of bronchi, bronchioles and parenchyma were significantly more in the test groups compared to saline and sham groups (P<0.001); also inflammation in pepsin and bile salts groups (histopathology scores: 2.87±0.35 and 3.0±0.0 for bronchial, 2.87±0.35 and 2.87±0.35 for bronchioles, 2.87±0.35 and 2.87±0.35 for parenchymal inflammation) were more than hydrochloric acid and gastric fluid groups (1.75±0.46 and 2.5±0.53 for bronchial, 2.0±0.0 and 2.0±0.0 for bronchioles, 2.0±0.0 and 2.0±0.0 for parenchymal inflammation) (P<0.05). The same results were found for fibrosis, so that the fibrosis in pepsin and bile salts groups were more than hydrochloric acid and gastric fluid groups (P<0.05). Conclusion: The present results suggested that pulmonary complications causing from bile salts and pepsin might be more than gastric juice and hydrochloric acid.