In a method of ventilating excised lungs, a ventilation gas is supplied to an airway of a lung and a vacuum is formed around the lung. A quality of the vacuum is varied between a lower level and a higher level to cause the lung to breathe, while the pressure of the ventilation gas supplied to the airway is regulated to maintain a positive airway pressure in the airway of the lung. The vacuum may be cyclically varied between the two vacuum levels. The levels may be maintained substantially constant over a period of time, or one or both of the lower and higher levels may be adjusted during ventilation. The lung may be placed in a sealed chamber, and a vacuum is formed in the chamber around the lung.