The major function of the lung is gas exchange and depends on alveolar and capillary parameters such as surface area and volume. The number of alveoli may report on the nature of structural changes in lung parenchyma during development, illness or changing environmental factors. We therefore developed an efficient and easily applicable stereological design for estimating and monitoring these structural parameters in the mouse lung. The estimation of volume fractions of different lung compartments has been carried out by point counting. A combination of cycloid grids superimposed on vertical sections was used to estimate the capillary surface area with isotropic test lines. Capillary length could be measured using the harmonic mean of the surface weighted diameter. The Euler characteristic applied in the physical fractionator with varying but known sampling fractions (Horovitz-Thompson estimator) enabled us to estimate alveolar number. In adult mice lungs, we obtained total values for alveolar number of 2.31 x 10(6) alveoli in a pair of lungs, alveolar surface area of 82.2 cm(2), capillary surface area of 124 cm(2), and capillary length of 1.13 km. All values are corrected for tissue shrinkage. With this study we present a highly efficient combination of several design-based stereological tools for the unbiased estimation of alveolar number and volume as well as length, surface area, and diameter of capillaries in the mice lung. Anat Rec, 292:113-122, 2009. (c) 2008 Wiley-Liss, Inc.