The influence of body size upon the decrease in weight-specific metabolic rate of organisms with increasing size is discussed. It is suggested that metabolic rate is closely correlated with, and perhaps dependent upon, the ability of an organism to obtain oxygen from the external environment. A model is presented that illustrates the advantage of having a decreasing weight-specific metabolic rate with increasing body size, rather than a linear relationship between the two. Support for the model is presented in the form of a literature review and with experimentation conducted with the aquatic salamander Siren lacertina.