Casting of silicone rubber replica casts of upper respiratory airways including nose, larynx, trachea, and tracheobronchial airways is described for the human and for standard laboratory animals. Morphometric measurements were made on nasopharyngeal and tracheobronchial airway casts. Cross-sectional areas of nasopharyngeal airways were observed to be complex in nature; each species exhibited different branching patterns and numbers of lobes. Replica casts were used to understand anatomical differences in nose and lung due to parameters such as age, sex, airway branching patterns, and morphometric dimensions. Effects of growth on morphometric dimensions were shown to be highly significant for use in extrapolating toxicological results from one age group to another. Species and strain differences in morphometric dimensions were also observed. Use of replica casts for velocity distribution pattern studies and for particle deposition efficiencies studies were investigated.