ObjectiveTo investigate the viscoelastic properties of lungs and thoracic wall in piglets. Study designProspective experimental study. AnimalsSix piglets weighting 30 kg. MethodsAnimals were tracheotomized, anesthetized and mechanically ventilated under controlled conditions. After control measurements of the mechanical properties of the lung of the pigs had been taken, acute lung injury (ALI) was induced by saline lavage. Lung and thoracic wall tissue resistance (ΔR), which reflects viscoelastic properties and/or time constant inequalities, were determined by using a rapid airway occlusion technique during constant flow inflation ( V˙), at constant tidal volume. V˙ was varied from 0.1–0.2 to 1.2 L second−1 on a single breath. Multiple data sets of ΔR of lung (ΔRL) and thoracic wall (ΔRw) to inspiratory time (TI = VT/ V˙) were fitted to a model whose prediction equation was ΔR = R2[1 −exp(−TI/τ2)], where R2 and τ2 are the ‘viscoelastic’ resistance and time constant, respectively. Subscripts L and W are used to represent lung and theracic wall, respectively (R2L, R2W, τ2L, τ2W). Two more sets of physiological measurements were then taken – the first under zero end-expiratory pressure (ZEEP) and the second under a positive end-expiratory pressure (PEEP) of 10 cmH2O. ResultsData of ΔR adequately fitted to the prediction equation in all instances. In control, R2,L was 15.3 (10.7–22.6) cmH2O L−1 second−1 (median, interquartile range), τ2,L 3.3 (1.9–5.5) seconds, R2,w 6.5 (2.2–10.3) cmH2O L−1 second−1 and τ2,w 2.9 (1.1–4.3) seconds. In ALI, R2,L significantly increased to 129.6 (105.9–171.3) cmH2O L−1 second−1 on ZEEP but not significantly decreased to 48.9 (17.8–109.6) cmH2O L−1 second−1 with PEEP. The corresponding values of τ2,L were 7.1 (5.1–11.6) and 4.4 (3.1–5.5) seconds. The values pertaining to thoracic wall did not change significantly among conditions. Conclusions and Clinical relevanceViscoelastic properties of the lung and thoracic wall in piglets can be described by a viscoelastic model. Values of parameters of this model were markedly increased in ALI and decreased with PEEP.