A method of calculating the volume of a tree distal to a cut at the origin of a branch, using branching, diameter and length ratios, has been developed. The method was applied to bronchial tree casts from human, dog, sheep, hamster, and rat lungs. It was found that the exponent a in the equation weight = k × diametera is approximately equal to 3.0 in sheep lung casts, as found by Hooper (1977) , but it is greater than 3.0 in casts from the other four species.