Objective: To investigate if bronchoalveolar lavage leads to increased alveolar and physiologic deadspace or a deadspace/tidal volume ratio and if surfactant replacement restores the lung to its prelavage condition. Design: Prospective, animal cohort study. Setting: An animal laboratory in a university medical center. Subjects: Seven adult rabbits receiving artificial ventilation. Methods: Our single-breath CO₂ analysis station contained the following equipment: pneumotachometer Ventrak 1550, a mainstream capnometer Capnogard 1265, a signal processor, and computer software. Repeated bronchoalveolar lavage was performed in seven adult rabbits to simulate acute respiratory distress syndrome. Surfactant therapy was administered after bronchoalveolar lavage induced a 20% reduction in baseline arterial PO₂. The calculated parameters of alveolar and physiologic deadspace and the deadspace/tidal volume ratio were derived from the single-breath CO₂ plot by Ventrak 1550 in combination with the Capnogard 1265. The arterial end-tidal PCO₂ difference, the alveolar-arterial PO₂ difference, and the arterial/alveolar PO₂ ratio were obtained by capnography and arterial blood gas analysis. Measurements of these parameters were performed before bronchoalveolar lavage, during bronchoalveolar lavage, and after surfactant application. Measurements and Main Results: The alveolar and physiologic deadspace and the deadspace/tidal volume ratio were significantly higher in lavaged animals. After application of natural surfactant, these parameters were significantly reduced but the baseline values could not be reached. Bronchoalveolar lavage led to a significant fall in the arterial/alveolar PO₂ ratio, which increased after surfactant therapy. There was a negative correlation between the arterial/alveolar PO₂ ratio and the deadspace/tidal volume ratio. The alveolar and physiologic deadspace and the deadspace/tidal volume ratio correlated with the arterial end-tidal PCO₂ difference. The best correlation was obtained between the arterial end-tidal PCO₂ difference and the alveolar deadspace/tidal volume ratio (r = 0.98). Conclusions: Bronchoalveolar lavage elevates the alveolar and physiologic deadspace and the deadspace/tidal volume ratios and is combined with a fall in the arterial/alveolar PO₂ ratio. Surfactant treatment improves the gas exchange but does not restore the lung to its prebronchoalveolar lavage condition, which indicates that the exogenous surfactant affects only partly the recruitment of the atelectatic areas.