Deposition data for a potentially susceptible subpopulation such as adolescents are needed, as well as dosimetry models that can account for variables important to a subpopulation. Since the nose acts as the first line of defense against inhaled airborne contaminants, the nasal structure of a number of Fischer-344 male rats of different age groups was investigated in the study. Replica casts of nasal-pharyngeal airways were made by injecting silicone rubber through the trachea, and sections in anterior-posterior positions were made for morphometric study. Comparison between the measured cross-sectional areas of the adult rat (body weight 167.2g and 365.8g) and the adolescent rat (body weight 12.1, 20.8, 38.7, 65.9, and 96.8 g) nasal airways showed a large number of similarities and dissimilarities. In the groups studied, the cartilage of the septum decreased as the age increased. The frontal nasal duct that enters the anterior part of the middle meatus grew as the age increased. In young noses, the sphenoidal recesses formed gradually. The ethmoidal and maxilloturbinates developed as a function of age.