The geometrical structure of the lung is one of the main factors governing inhaled particle deposition; structural differences among different species are, therefore, of great importance for extrapolation modeling. A statistical analysis of morphometric data for the human and rat tracheobronchial tree reveals significant interspecies differences in airway branching patterns: compared to the relative dichotomous and symmetric structure of the human lung, the rat lung displays a more monopodial airway branching pattern. Thus, for the rat lung a given airway is characterized, its size, physiologic function, and distance from the trachea by its diameter and not by a theoretically assigned generation number. A Monte Carlo method is used to construct an airway geometry along each inhaled particle's path by selecting airway parameters randomly from their frequency distributions and the correlations among them. While the airway geometry is selected randomly, particle deposition in individual airways is calculated analytically. (Copyright (c) 1989 Health Physics Society, Pergamon Press plc.)