The relationship between basal metabolism P and body mass M of 391 mammalian species has been analysed by least-squares regression, robust regression and covariance analyses. This relationship is a power function: P = aMb, where the mass exponent b is 0.678 +/− 0.007 (mean +/− S.D.) and the mass coefficient a takes different values. Theory of measurement revealed that the 2/3 mass exponent is due to an underlying dimensional relationship between the primary quantity of mass and the secondary quantity of power. This paper shows that the 2/3 mass exponent is not the physiological problem of interest. It is not the slope of the metabolic regression line, but its location in the mass/power plane, that must be explained. This location is given by the value of the mass coefficient, the explanation of which is, and remains, the central question in comparative physiology.

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