The relationship between gas trapping in excised rat lungs and pulmonary edema was studied. Pulmonary edema was induced in male Long-Evans-rats by means of high inspiratory positive pressure breathing. The lungs were then inflated and deflated for six different sequences of four cycles each. Deflation was at an end expiratory pressure of positive 6, 4, 3, 2, 0, or negative 5 centimeters of water. The volume of gas trapped in the lungs was measured. Lungs with pulmonary edema trapped gas at more positive values of end expiratory pressure than normal lungs. The authors conclude that at more positive transpulmonary pressures, the airways in excised lungs close sooner than those in normal lungs.