In the house sparrow, the budgerigar and the violet-eared hummingbird the volumes of the lungs and air sacs are estimated from silicone casts. The quantitative composition of the lungs and of their compartments are measured on lung slices, the relative volumes of the parabronchi on histological sections, and the volume composition of the blood-air capillary network of the parabronchi on electron micrographs. On electron micrographs the exchange surface and the thickness of the air-blood diffusion barrier are also measured. From these data the morphological membrane diffusion capacity is calculated and related to several organ weights. The volume of the lungs and air sacs makes up 14-22% of the total body volume, the lungs only 2.3-2.9%. The exchange surface varies from 61 cm2/g (budgerigar) over 70 cm2/g (house sparrow) to 99 cm2/g (violet-eared hummingbird). The very thin barrier in these small birds results in a membrane diffusion capacity of 0.122 in budgerigars up to 0.271 ml O2/mm Hg . min . g in violet-eared hummingbirds. The various parameters are compared with those of corresponding mammals, and the quantitative advantages of the avian respiratory system are discussed.