By intratracheal injection of the protease papain to experimental animals parenchymal changes in the lung can be induced, that resemble human emphysema. Papain (dosage 26 to 112 mg, 1 to 4 injections) was given intratracheally to 8 bastard dogs (weighing from 12.5 to 20 kg) during light general anesthesia. Pulmonary function was assessed in weekly intervals and related to morphologic changes in the lung. Static compliance of the lung and FRC measured during respiratory arrest were increased after papain, bronchial resistance, measured while artifically ventilated at constant pressure was also increased. Changes of static lung compliance and FRC were seen after the first administration of papain, but further increased with time of observation and after multiple doses of papain. Increase of resistance was not found before 5 weeks. At quiet breathing resistance was not increased at all. No significant changes were found of arterial pO2 and pCO2, pH, standard and actual bicarbonate, diffusion capacity for O2, tidal volume, minute ventilation and ventilatory rate. Morphological findings confirmed the changes described by others. Pulmonary function appears to be pathological at a time when morphology still seems to be normal. The question is discussed to what extent the model of experimental emphysema induced by proteolytic enzymes can contribute to the understanding of human pulmonary emphysema. Lung function in the course of experimental emphysema is compared with function in different clinical types of emphysema.