The present study demonstrates the beneficial effects of PEEP on the functional behavior of rabbit lungs after five hours anaesthesia with halothane and methoxyflurane. Instead of a highly significant decline of lung compliance as was seen after spontaneous breathing and IPPB, PEEP led to a constant though statistically non-significant rise of this criterion. Blood gas values, too, demonstrated clearly the superiority of this ventilatory pattern. In contrast to former investigations there were no histological findings indicating any structural damage to the lung. Postanaesthetic examinations of lung extracts in the Wilhelmy balance showed no deterioration after PEEP. From these experimental findings it is concluded that patients suffering from surfactant deficiencies will benefit from a moderate PEEP during anaesthesia with halothane or methoxyflurane.