The thickness of the air-blood barriers in the lungs of a series of mammals, reptiles and amphibians was estimated using stereological methods. There was considerable variation in the measurements, the thickest barrier being that of the crested newt (2.81 micrometer) and the thinnest that of the mouse (1.27 micrometer). The arithmetic mean thickness of the barriers was related to body weight in mammals but not in reptiles or amphibians. There were no significant differences between the minimum thickness of the barriers nor in the volume proportions of their constituent layers in the three orders of animals. The ratio of the arithmetic mean thickness to the harmonic mean thickness of the barriers was highest in mammals. These data indicate that the process of optimization of lung architecture for gas exchange is most marked in mammals.