BACKGROUND: Airway secretions are a source of complications for patients with acute and chronic lung diseases, yet lack of techniques to quantitatively track secretions hampers research into clinical measures to reduce their pathologic consequences. METHODS: In a preserved swine lung model, we tracked a contrasted mucus simulant (CMS) using sequential computed tomography (CT). Known drivers of secretion movement - gravity and ventilation - were tested. Ten millilitres of CMS were unilaterally introduced (1 ml min(-1)) into the airways of 12 lung sets. After instillation, six lung sets were maintained prone and six were rotated 180 degrees . Subsequently, all were mechanically ventilated for 10 min. CTs were obtained before infusion, after infusion and after ventilation +/- rotation. For CT analysis, the lungs were partitioned into eight sub-cuboids using anatomic landmarks. The volumes of two CT number ranges representing CMS and poor aeration/collapse were computed in every sub-cuboid for each CT acquisition. Volume differences between study time points were used to quantify changes. RESULTS: CMS and poor aeration/collapse volume change distributed gravitationally after infusion. After ventilation without rotation, the CMS and poor aeration/collapse volumes remained within the originally injected sub-cuboid, although the poor aeration/collapse volume expanded (27.3 +/- 6.1-->50.5 +/- 7.4 ml, P<0.05). After ventilation + rotation, there was a reduction in the CMS and poor/aeration collapse volumes in the originally injected sub-cuboid (14.4 +/- 1.7-->4.4 +/- 0.6 ml, P<0.05 and 18.3 +/- 3.8-->11.9 +/- 2.7 ml, P<0.05, respectively) accompanied by increases in the gravitationally opposite sub-cuboid (1.7 +/- 0.2-->11.1 +/- 1.1 ml, P<0.05 and 0.8 +/- 0.5-->40.6 +/- 3.5 ml, P<0.05, respectively). CONCLUSION: Movement of fluids within the bronchial tree can be semi-quantitatively tracked with analysis of sequential CT acquisitions. In this isolated swine lung model, gravity had an important and brisk effect on movement of a viscous fluid, whereas ventilation tended to embed it peripherally.