Regional viscoelastic properties of thoracic tissues in acute respiratory distress syndrome (ARDS) and their change with position and positive end-expiratory pressure (PEEP) are unknown. In an experimental porcine ARDS, dorsal and ventral lung (R(2),L and E(2),L) and chest wall (R(2),cw and E(2),cw) viscoelastic resistive (R) and elastic (E) parameters were measured at 20, 15, 10, and 5 cmH(2)O PEEP in supine and prone position. E(2) and R(2) were obtained by fitting the decay of pressure after end-inspiratory occlusion to the equation: P(viscmax) (t) =R(2) e(-t/τ(2)), where t is the length of occlusion and τ(2) time constant. E(2) was equal to R(2)/τ(2). R(2),cw and E(2),cw were measured from esophageal, dorsal, and ventral pleural pressures. Global R(2),L and E(2),L were obtained from the global transpulmonary pressure (airway pressure-esophageal pressure), and regional R(2),L and E(2),L from the dorsal and ventral airway pressure-pleural pressure difference. Lung ventilation was measured by electrical impedance tomography (EIT). Global R(2),cw and E(2),cw did not change with PEEP or position. Global R(2),L [median(Q1-Q3)] was 37.1 (11.0-65.1), 5.1 (4.3-5.5), 12.1 (8.4-19.5), and 41.0 (26.6-53.5) cmH(2)O/L/s in supine, and 15.3 (9.1-41.9), 7.9 (5.7-11.0), 8.0 (5.1-12.1), and 12.9 (6.4-19.4) cmH(2)O/L in prone from 20 to 5 cmH(2)O PEEP (P = 0.06 for PEEP and P = 0.06 for position). Dorsal R(2),L significantly and positively correlated with the amount of collapse measured with EIT. Global and regional lung and chest wall viscoelastic parameters can be described by a simple rheological model. Regional E(2) and R(2) were uninfluenced by PEEP and position except for PEEP on dorsal E(2),L and position on dorsal E(2),cw.NEW & NOTEWORTHY In a porcine model of acute respiratory distress syndrome, data were successfully fitted to a rheological model of the nonlinear behavior of viscoelastic properties of lung and chest wall at different positive end-expiratory pressure (PEEP) in the supine and prone position. Prone position tended to decrease lung viscoelastic resistive component. PEEP had a significant effect on dorsal lung viscoelastic elastance. Finally, lung viscoelastic resistance correlated with the amount of lung collapse assessed by electrical impedance tomography.