Mechanical properties of the proximal airways are known to change with development; the highly compliant airways of the immature animal become stiffer and less collapsible with increasing age. Although the relationship between tracheobronchial architecture and function has been described for adult physiology, little is known regarding this relationship during early development. This study was, therefore, designed to test the hypothesis that alterations in tracheal morphometry parallel developmental differences in tracheal functional properties. Tracheal segments obtained from 29 lambs ranging in age from 70% of gestation to full-term newborn lambs up to 6 d old were examined using anatomic, morphometric, and histochemical techniques. The results showed 1) progressive increases in the dimensions of the trachea and the tracheal wall components, 2) alterations in the geometric arrangement of the tracheal ring, and 3) changes in the compositional characteristics of the tracheal cartilage with maturation. These findings demonstrate alterations in tracheal architecture, each of which contribute to the greater stiffness of the trachea, in older animals. When considered together, these factors help explain the differences in tracheal functional characteristics with development.