A realistic two-dimensional model of aerosol transport and deposition in the alveolar zone of the human lung was developed, using, the FIDAP software package (Fluent, Inc). Gas flow corresponding to a mouth flow rate of 500 ml s(-1) was computed in a symmetric 6-generation structure of alveolated ducts located in a vertical plane. A bolus of 2 mum-diameter particles was introduced in the model at the beginning of inspiration. Particle trajectories were predicted for one breath cycle (2 s inspiration, 2 s expiration). There were large non-uniformities in deposition between generations, between ducts of a given generation and within each alveolated duct, suggesting that local aerosol concentrations can be much larger than the mean acinar concentration. A significant number of particles failed to exit the structure during expiration. Most of these particles were located deep in the structure (distal to the 3rd generation), leaving them in a position to penetrate deeper in the lung during the subsequent inspiration and eventually deposit.