Surface tension forces acting on the thin-walled alveolar septa and the collagen-elastin fiber network are major factors in lung parenchymal micro-mechanics. Quantitative serial section analysis and morphometric evaluations of planar sections were used to determine the spatial location of collagen and elastin fibers in Sprague Dawley rat and normal human lung samples. A large concentration of connective tissue fibers was located in the alveolar duct wall in both species. For rats, the tissue density of collagen and elastin fibers located within 10 micrometers of an alveolar duct were 13 and 9% respectively. In human lung samples, the tissue density of collagen and elastin fibers withon 20 micrometers of an alveolar duct were 18 and 16% respectively. In both species, bands of elastin fibers formed a continuous ring around each alveolar mouth. In human lungs elastin fibers were found to penetrate significantly deeper into alveolar septal walls than they did in rat lungs. The concentration of connective tissue elements in the alveolar duct walls of both species is consistent with their proposed roles as the principal load bearing elements of the lung parenchyma. (Copyright (c) 1990 the American Physiological Society.)