We present and document an hypothesis that healthy adults of most vertebrate species use 2-8% of their basal metabolism for the central nervous system (CNS). This relationship is constant across all classes of vertebrates, as we found by examining data from 42 species, including 3 fish, 3 amphibia, 2 reptiles, 6 birds, and 28 mammals. To explain its constancy, we hypothesize that an optimal functional relationship between the energy requirements of an animal's executor system (muscle metabolism) and its control system (CNS metabolism) was established early in vertebrate evolution. Three types of exceptional cases are discussed in terms of the hypothesis: very large animals, domesticated animals, and primates.