Extrapolations of biological responses obtained in laboratory animals to human exposure conditions are commonly based on local particle deposition patterns. Although laboratory rats are frequently used in inhalation toxicology as human surrogates, their airway system exhibits a much more asymmetric branching pattern than that of the human lung. To investigate the effect of this structural difference on particle deposition, local particle deposition patterns were plotted as functions of three different morphometric parameters: airway generations, airway diameters, and cumulative pathlengths. Our search of the most appropriate morphometric parameter to characterize local particle deposition for extrapolation modeling purposes was based on two criteria: (i) the similarity of the distribution of deposited particles within the lungs, and (ii) the similarity of intrasubject variability in deposition in individual airways. The results presented in this paper suggest that airway diameters are a more appropriate morphometric parameter to classify local particle deposition patterns across the two species than the commonly used airway generations.