The present study demonstrates that repetitive recruitment manoeuvres are not only beneficial for lung functions, gas exchange and barrier function, but also reduce the liberation of pro-inflammatory cytokines (Fig. 5) . This is an important finding, because in theory frequent deep inspirations might be a trigger for stretch-induced pro-inflammatory mediator release. IL-6 and KC were increased in the BAL and blood serum of all ventilated mice and it appears that ventilation will always cause mild inflammation in the lungs, both experimentally [7, 8, 25] and clinically [45, 46] . The underlying mechanisms may be related to the surgery or mechanotransductive processes, i.e. stretchactivation of signalling cascades, or to increased vascular shear stress in those groups ventilated with higher positive pressures [47] . IL-6 and KC were significantly lower in the BAL fluid of lowV T RM5 mice compared to all other groups, demonstrating that this ventilation strategy was the most protective one. It is well-established that protective ventilation, minimizing overdistension and recruitment/derecruitment of the lung, attenuates cytokine liberation in the lung and circulation [48] . Interestingly, the blood serum of both V T groups ventilated with RM5 contained significantly lower levels of IL-6 and KC than the corresponding groups ventilated with RM60 or noRM. This shows that the RM5 procedure was also protective at higher tidal volumes. The high levels of IL-6 and KC in the BAL of highV T RM5 mice indicate that the inflammatory response was initiated in the lung. Notably, neither levels of IP-10 nor of TNFa -two cytokines that are frequently found in severe forms of acute lung injury -were altered significantly in ventilated mice, compared to unventilated controls. The relatively high baseline values for TNF-a in unventilated mice are most likely due to the fact that BAL samples had to be diluted on order to gain enough volume for the assay. Concentrations were calculated by multiplication with the dilution factor, possibly leading to higher concentrations than actually present in the BAL. Nevertheless, all groups were analyzed at the same time and cytokine levels were in the same range in all groups, allowing a comparison. IP-10 is proposed to play a role in development of ARDS and is further considered as a useful biomarker for lung disease [49, 50] . TNF-a is considered as a mediator of pulmonary inflammation released by injurious ventilation [51, 52] . The finding that these mediators remained unaltered by ventilation further suggests that lung injury was only moderate in the current model. This study aimed to define settings for ventilation under stable conditions and therefore a second set of experiments was performed with a PEEP of 6 cmH 2 O, to address the question whether a higher PEEP could prevent the severe impairment of lung functions and the effects on oxygenation and inflammation, which were observed in the lowV T noRM and lowV T RM60 groups ventilated with a PEEP of 2 cmH 2 O. Lung mechanics and blood gas results revealed that a higher PEEP, even when combined with RM60, was not sufficient to prevent formation of atelectasis. C decreased and R, G and H increased strongly during the first 180 min of the experiments in the PEEP6_noRM group before parameters reached a plateau. A previous study demonstrated that an increase in PEEP from 2 cmH 2 O to 6 cmH 2 O alone did not prevent an increase in R aw , G and H during 150 min of ventilation, but was more effective during pressure controlled RM [5] . The present study shows that a combination of higher PEEP and short RM every 60 minutes does not suffice to prevent impairment of lung functions and that frequent RM are required, as with a PEEP of 2 cmH 2 O. Of note, the increase in R aw , which was observed in the groups lowV T noRM and lowV T RM60 ventilated with a PEEP = 2 cmH 2 O, was completely abolished in all groups ventilated with a PEEP = 6 cmH 2 O, indicating that a higher PEEP helps to prevent narrowing of large airways during MV.