This article will describe a method for perfusing and ventilating rat lungs which allows assessment of lung mechanics, segmental vascular resistance, gas exchange, and vascular permeability. Isolated perfused rat lungs (IPL) were ventilated by negative pressure ventilation and were perfused at constant pressure with recirculating, albumin-containing Krebs-Henseleit buffer. A newly constructed respirator pump facilitated well-defined ventilation of the lungs and permitted switching between negative and positive pressure ventilation. Using this setup lungs were ventilated with positive pressure during both surgery and measurement of lung weight. Once the lungs were prepared, electronic control circuits ensured stability of the perfusate pH and perfusate pressure. Such preparations were stable for at least 3 hr. Breathing mechanics, for example, tidal volume, pulmonary compliance, and pulmonary resistance, perfusate characteristics, such as pulmonary vascular resistance, pulmonary pre- and postcapillary resistance, perfusate pH, PO2, and PCO2, and the capillary filtration coefficient were determined. All measured parameters were within normal physiological range, and the lungs were biochemically active, and responded to various stimuli.