An O3 dosimetry model is used to simulate the local absorption of O3 in the lower respiratory tract (LRT) of rats and guinea pigs. The model takes into account LRT anatomy, transport in the lumen and air spaces, transport and chemical reactions in the mucous and surfactant layers and in the underlying tissue and capillaries. For each species two anatomical models were used to investigate their influence in predicting absorption. Results with all four anatomical models and various ventilatory parameters showed a qualitative similarity in the shape of the dose versus airway number curves but significant differences in predicted percent total and percent pulmonary uptake. The percent uptake was also sensitive to breathing frequency and tidal volume. Rat lobe models were used to study absorption in lobes and show that O3 tissue dose in centriacinar regions decrease with increasing distance from the trachea. The effect on results of values used for functional residual capacity (FRC) and of values used for the chemical rate constants for O3 reactions in mucus were explored. Results differed quantitatively but not qualitatively.