Studies were conducted to determine the effects of minimum transpulmonary pressure (Pmin), maximum transpulmonary pressure (Pmax), and inflation/deflation rate on the amount of trapped air accumulated in the excised rat lung during ventilation. Degassed excised rat lungs were ventilated in a plethysmograph filled with saline and water; a ventilation (inflation/deflation) cycle was produced by drawing saline out of the apparatus, which produced inflation of the lungs and then pumping the saline back in which produced deflation of the lungs. Measurements were taken for 10 consecutive cycles. Pressure/volume curves were recorded from a Pmin of minus 5cm H2O 5cm H20 to a Pmax of 30cm H2O. Amount of air trapped in the lung was taken as an index of the minimum volume of the lung (Vm) divided by the maximum volume of the lung (Vmax) for one complete cycle. Inflation/deflation rate was kept at 3.82ml/min to determine the effects of limiting Pmax on the normalized Vm. Significant increases in the amount of air trapped in the lungs was observed as the flow rate was reduced from 38ml/min to 1.9ml/min. When Pmax was reduced to 25cm H20 or 20cm H20 or Pmin was increased to 6cm H2O or 11cm H2O, the amount of trapped air significantly decreased. Inflation rate was found to affect the amount of trapped air more than deflation rate or a stress adaptation period at Vmax. The amount of trapped air could not be reduced by having a cycle with a rapid inflation rate follow a cycle with a slow inflation rate, which suggested that the accumulation of trapped air is an additive process. It is concluded that the results are consistent with the theory that bubbles formed during inflation are the main cause of air trapped in the excised lung.