Respiration rate (R ) as a function of body mass (W  ) is usually expressed as R = aW b. Empirically, the b value is often close to 3/4 when organisms covering a large span in body mass are compared. But recent years research on the energetic cost of growth demonstrate that young and fast growing stages show higher weight specific respiration rates than older and adult stages, and this implies that the b values tend to be higher: b∼ 1 in small (young) organisms falling to b = 0.6–0.7 in larger (older) stages. Thus, respiration and growth are integrated through the energetic costs of growth. This explains why the b value is not a “natural constant” and why a “3/4 power scaling law” cannot be deduced from the interplay between pure physical and geometric constraints of the transport of oxygen.