In fact, lung volume declined only to a certain threshold and a plateau phase was reached in all mice ventilated without RM. This supports the hypothesis that depending on the pressure applied to the lung some populations of alveoli are open and others closed [38] . The minimal lung volumes in the plateau phase differed according to the employed tidal volume, leading to the highest tissue resistance and elastance values in the lowV T noRM group. Formation of atelectasis was more severe in low V T than in high V T ventilation, as shown elsewhere [3, 4] , indicating that low V T ventilation without RM was injurious. These findings are in line with a previous work showing that low V T ventilation augmented lung injury in isolated rat lungs, proposing that the degree of lung injury is dependent on the end-expiratory lung volume [41] . Ventilation with high V T resulted in a higher compliance and thereby to some degree protected from the decline in lung volume. However, the application of one RM per hour was not sufficient to keep tissue resistance and elastance entirely stable over six hours, demonstrating that the beneficial effect of recruitment is transient and formation of atelectasis is reversible only to a certain point. The present data emphasize the importance of frequent deep inflations in mice in order to maintain lung functions in a physiological range.