The influence of pharmacological stimulation of pulmonary surfactant synthesis has been studied in rat alveolar spaces. Animals were treated acutely with Ambroxol at a dose of 4 mg/kg b.w./day p.o. and 5 days later the following biochemical and physico-chemical parameters were determined: BAL fluid lecithin content, BAL fluid microviscosity, alveolar macrophage membrane microviscosity, spontaneous generation of superoxide anion by alveolar macrophages, elastase and antielastase activity of BAL fluid. Treatment with Ambroxol significantly increased the lecithin content of BAL fluid and significantly decreased the macrophage plasma membrane microviscosity. A likely consequence of increased lecithin content in alveolar macrophages (an activation of these cells) was suggested by the increase of the spontaneous production of superoxide. Finally, in the BAL fluid of Ambroxol-treated rats the elastase activity was reduced, whereas the elastase inhibitory activity was almost doubled in respect to control rats.