An experimental model of pulmonary interstitial fibrosis was attempted in beagle dogs by intravenous administration of weak doses of paraquat. After various trials, the final protocol involved the intravenous injection of 2 mg/kg repeated every eight days for three weeks, and then every 15 days. On the 70th day, areas of localized interstitial fibrosis were observed (less than 10% of the surface of the tissue sections) in an otherwise healthy parenchyma. There was no marked hepatic or renal lesion. The evolution of the inflammatory and toxic processes was followed by the changes: 1) in the animal's weight (drop in weight after the injection of paraquat), 2) in blood gases (essentially drop in PaO2 and in SaO2), 3) in ventilation and respiratory exchanges (increase of VE and ventilatory frequency, decrease of Duco2), and 4) in static pulmonary compliance (distinct fall after injection). All these modifications showed a close relationship to the pattern of the paraquat injections. They receded 15 days after the last injection and reappeared after a new injection. The pulmonary hemodynamic study carried out only at the beginning of the experiment and before the animal was killed showed only a few anomalies (reduction of the average pressure of the right atrium).