The events leading to the onset of the experimental bleomycin-induced pulmonary fibrosis are so far unknown, though recent observations emphasize the crucial role played by lung phospholipids and by alveolar macrophages. In an attempt to verify this point, a series of studies were undertaken by treating rats (CD-COBS) with intratracheal instillation of a single dose of bleomycin (1.5 U). At the same time a group of animals was pretreated with ambroxol (which is known to be a powerful inducer of surfactant production both in fetal and adult type II pneumocytes), 4 mg/kg body weight/day per os, 5 days before the treatment with bleomycin and up to the sacrifice of the animals at the 1st, 3rd, 7th, 14th or 28th day from the instillation. In our experimental model, the 14th day from the treatment with bleomycin seems to be a crucial time since it histologically corresponds to the proliferation of type II pneumocytes; furthermore, an increase of total lecithins in the alveolar spaces was observed, together with an increase of macrophage membrane fluidity. In the ambroxol-pre-treated group a partial reduction of the rate of interstitial fíbrosis was observed. At the 14th day from treatment the alteration of phospholipid levels was much less pronounced and the microviscosity of alveolar macrophages was similar to that of control animals. These data suggest that the onset of bleomycin-induced pulmonary fibrosis in the rat is characterized by an increase of alveolar lecithins and that this fact could modify the physico-chemical peculiarities of alveolar macrophages. Ambroxol shows a partially protective effect, perhaps by modulating the activity of type II pneumocytes