Morphometric and sterological methods were employed to evaluate the anatomical gas-exchange potential of the lungs of the fresh-water turtle, Pseudemys scripta elegans. The total lung volume and lung wet weight increase with the 1.4 power of body weight. Right and left lungs are similar in size and shape, containing a tissue-free central lumen, which occupies 55 per cent of the maximally inflated lung. The remainder is parenchyma, composed of 90 per cent air and 10 per cent tissue, with an effective surface-to-volume ratio of 18 cm-1. The anatomical diffusion factor (ADF), or the ratio of respiratory surface area to mean diffusion distance in lung tissue, is 1 order of magnitude lower than in the mammalian lung. Taking interspecific differences in O2 consumption into account, the degree to which the lungs are exploited, deltaPtO2, is similar in Pseudemys and in mammals at basal, resting and exercising states.