Silicone rubber casts of the respiratory tract were used in morphological studies of the human, baboon, rhesus monkey, dog, rabbit, guinea pig, rat, hamster, and mouse. In the studies, the trachea of the specimen was opened by tracheotomy, and silicone rubber (734 RTV) was introduced through the trachea to form nasopharyngeal and tracheobronchial casts. Measurements were made on the nasal structures, and the lungs were observed for species variation in branching pattern and number of lobes per lung. While species differences in respiratory tract anatomy are known to exist, the study provides a focus for toxicologists when extrapolating toxicological results from one species to another.