Background High positive end-expiratory pressure (PEEP) and low tidal volume (VT) ventilation is thought to be a protective ventilation strategy. It is hypothesized that the stabilization of collapsible aiveoli during expiration contributes to lung protection. However, this hypothesis came from analysis of indirect indices like the analysis of the pressure-volume curve of the lung. The purpose of this study was to investigate isolated healthy and injured rat lungs by means of alveolar microscopy, in which combination of PEEP and VI- is beneficial with respect to alveolar stability (I-E%).Methods Alveolar stability was investigated in isolated, non-perfused mechanically ventilated rat lungs. Injured lungs were compared with normal lungs. For both groups three PEEP settings (5, 10, 20 cmH_2O) were combined with three VT settings (6, 10, 15 ml/kg) resulting in nine PEEP-VT combinations per group. Analysis was performed by alveolar microscopy.Results In normal lungs alveolar stability persisted in all PEEP-VT combinations (I-E% (3.2±11.0)%). There was no significant difference using different settings (P >0.01). In contrast, alveoli in injured lungs were extremely instable at PEEP levels of 5 cmH_2O (mean I-E% 100%) and 10 cmH_2O (mean I-E% (30.7±16.8)%); only at a PEEP of 20 cmH_2O were alveoli stabilized (mean I-E% of (0.2±9.3)%).Conclusions In isolated healthy lungs alveolar stability is almost unaffected by different settings of PEEP and VT. In solated injured lungs only a high PEEP level of 20 cmH_2O resulted in stabilized alveoli whereas lower PEEP levels are associated with alveolar instability.