A morphologic description of the airways of the guinea pig was developed from measurements of casts of the lungs and nasal cavity and from measurements of frozen sections of the lungs. The lengths, diameters, branching pattern, and numbers of elements of the respiratory tract formed the basis for a representative model of the system. The brainching pattern is irregular to the pulmonary region but regularly dichotomous thereafter. The nasopharyngeal-tracheobronchial region contributes 2.64 cm3 of the total respiratory volume of 21.62 cm3. The alveoli contribute 16.31 cm3 of the 18.98 cm3 pulmonary region. The nasal region consist of convoluted and irregular airways with a functional volume of 0.48 cm3.