We studied the effect of gravitational sedimentation on the dispersion of 0.5 and 1 mum-diameter particle boluses within a two-dimensional symmetric six-generation model of the human acinus. Boluses were introduced at the beginning of a 2-s inspiration immediately followed by a 4-s expiration, in normal gravity (1G) and in the absence of gravity (0G). The flow corresponded to a flow rate at the mouth of 500 ml/s. In 0G, simulated dispersion (H-sim) was 16 ml for both particle sizes. In 1G, H-sim, was 71 and 242 ml for 0.5 and 1 mum-diameter particles, respectively, showing the effect of gravitational sedimentation. The difference between experimental data (J. Appl, Physiol. 86 (1999) 1402) and simulations was independent of particle size. This suggests that the residual dispersion was independent of the intrinsic properties of the particles and was more likely due to other mechanisms such as ventilation inhomogeneities, cardiogenic oscillations and alveolar wall motion. (C) 2003 Elsevier Science Ltd. All rights reserved.