SYNOPSIS. Standard metabolic rates are principally determined by relatively few factors: temperature, mass, and phylogenetic affinities. 1) Temperature. Body temperature has pervasive effects on biological processes through the influence of temperature on the activity of metabolically important enzymes: metabolic rates double or triple when body temperature increases 10°C. The absolute levels of metabolism that any temperature establishes may be subject to acclimatory or evolutionary change by alteration of enzyme composition or structure. In endotherms, the effects of this basic thermal dependence may be overcome by the physiological regulation of body temperature, but such regulation is very energetically expensive. 2) Body mass. Metabolic rate scales interspecifically with body mass0.75. We understand to a degree how animals are structured to produce this observed allometric dependence of metabolism on mass, principally by quantitative differences in cellular metabolism that can be attributed to differential mitochondrial content. But we do not understand why animals are so structured or what exactly is the reason for this particular mass exponent. 3) Phylogeny. Metabolic level varies among taxa, and the phylogenetic history and relative associations of an animal are important determinates of energy expenditure. Mammals and birds have substantially greater metabolic rates than do ectothermic vertebrates, even at common body temperatures. However, even within vertebrate classes, metabolic level varies among orders. The principle of symmorphosis has been proposed to describe the structural consequences of functional design and the morphogenetic response of biological systems to functional need.