Data obtained from animal exposures may, if appropriately extrapolated, be used to assess quantitatively the health risks of humans chronically exposed to environmental airborne pollutants. The ability to make correct extrapolations is critical in making full use of the results of acute and chronic exposures in laboratory animals. Extrapolation of experimental results between different animal species requires several types of data, one of which is detailed data on the geometry of the lungs for each species, to determine the quantity of pollutant that reacts (or is deposited) in the various anatomic regions (trachea, bronchi, bronchioles, terminal bronchioles, proximal alveolar and distal alveolar regions). Data on the geometry of the lungs are almost exclusively concerned with the geometry of the major airways. The concentration of previous work on the major airways reflects the fact that they are the major determinants of airway resistance.