Lung development disruption in childhood due to air pollution is gaining growing attention. Although rats are widely used for studies of pulmonary toxicology, little is known about the fate of inhaled particles in juvenile rat lungs. We investigated detailed particle deposition in rat lungs at 15, 28, 40, and 81 days of age using a single path mathematical model. For normal breathing conditions, particles smaller than 0.1 mu m deposited in extrathoracic airways more in juvenile rats compared to adults whereas alveolar deposition fraction for juvenile rats was smaller compared to adults for most particle sizes. Alveolar deposition per minute per unit body mass was appreciably lower for juvenile rats compared to adults and deposition per minute per unit area was lower for juvenile rats over the entire lungs for most particle sizes. (C) 2011 Elsevier Ltd. All rights reserved.