The lungs of five female domestic Muscovy ducks, mean body weight 1.627 kg, total lung volume 48.07 cm3, were analysed by standard morphometric methods. Principal results obtained are: lung volume per unit body weight, 30.17 cm3/g; volume densities of exchange tissue relative to lung volume, 49.24%, blood capillaries relative to exchange tissue, 29.63%, tissue of the blood gas (tissue) barrier relative to exchange tissue, 5.88%; surface area of the blood-gas (tissue) barrier per unit body weight, 30.04 cm2/g; ratios of the surface area of the blood-gas (tissue) barrier per unit volume of the lung and per unit volume of exchange area, 979 cm2/cm3 and 200.06 mm2/mm3, respectively; harmonic and arithmetic mean thicknesses of the tissue barrier, 0.199 μm and 0.303 μm, respectively. The anatomical diffusing capacity of the tissue barrier for oxygen (DtO2) and the total pulmonary diffusing capacity (DLO2), 49.58 ml O2/min/mmHg/kg and 4.55 ml O2/min/mm Hg/kg, respectively. The lungs of the domestic Muscovy duck appear to be about as well adapted anatomically for gas exchange as the lungs of wild anatid species, and there is no clear evidence that domestication has been associated with any deterioration in the anatomical capacity for oxygen uptake. The weight-specific anatomical diffusing capacity of the lung for oxygen (DLO2/W) was about 3.6 times greater than the weight-specific physiological value, a factor which falls within the expected range.