The lungs of 5 domestic fowls have been analysed electron microscopically by standard morphometric methods. The anatomical diffusing capacity has been calculated from the relevant parameters. The blood-gas barrier consisted essentially of endothelium, basal lamina, and a very thin squamous epithelial cell. Granular cells are absent and interstitial tissue is minimal. The following measurements are mean values applying to both lungs together, in the fixed state: lung volume, 25.0 cm3; total intrapulmonary blood volume, 6.9 cm3; volume of the exchange tissue, 11.6 cm3; pulmonary capillary blood volume, 3.51 cm3; pulmonary capillary haematocrit, 62.7%; total intrapulmonary air volume, 16.61 cm3; air capillary volume, 6.68 cm3; surface area of the blood-gas barrier, 2.08 m2; surface area of the barrier per unit volume of exchange tissue, 179.5 mm2/mm3; arithmetic mean thickness of the tissue barrier, 1.20 micrometers; harmonic mean thickness of the tissue barrier, 0.314 micrometers; harmonic mean thickness of the plasma layer, 0.342 micrometers. The values of the maximum and minimum morphometric diffusing capacities for the lung (DL02 m) were respectively 3.55 and 1.50 ml O2/min/mm Hg; those for the membrane (Dmo2 m) were 13.61 and 12.01 ml O2/min/mm Hg.