This species comparative study examined tissue shrinkage from a known physiologic lung volume through to the processed histologic section. Eleven mammalian species with body weights that spanned 3 orders of magnitude were studied. Air pressure-volume curves were determined to obtain total lung capacity (TLC) at 30 cmH2O. The lungs were then fixed by airway filling at 25 cmH2O pressure, and a displacement fixed lung volume was determined. Linear dimensions were systematically measured on fixed tissue blocks, embedded tissue blocks, and stained sections. Results indicated that the ratio of fixed lung volume to TLC ranged between 0.6 and 2.0. The corresponding ratios for linear dimensions ranged between 0.8 and 1.3 for all species. Histologic processing caused further shrinkage; the ratios of linear dimensions measured after and to those measured before processing ranged between 0.6 and 0.7. Thus, the degree of fixed lung volume achieved relative to TLC varies considerably more among species than does the histologic shrinkage caused by processing. We conclude that measurement of these changes in lung dimensions caused by fixation and histologic processing in the different species is essential, particularly in quantitative interspecies physiologic studies.