The concept of optimal mechanical efficiency as a valuable adaptation in organic evolution leads logically to the measurement of anatomical structures with a view to ascertaining their degree of correspondence to a theoretically optimal design. This idea is here considered as it applies to samples of specimens from human cardiovascular and respiratory systems. It is shown that theoretical criteria of efficiency will indeed predict the mean measurements of the samples with a very high degree of accuracy. In individual cases, however, the correlation between theoretically predicted and actual observedvalues is very poor. The diffeculties inherent in isolating a small part of a total biological system and treating it as a controlled physical system are discussed, as are some of the biological variables which must limit many of the theoretical formulations. It is concluded that until further biological and statistical refinements are possible the present theoretical methods may have only limited practical application.