This series of four papers investigates the link between the energetics and the mechanics of terrestrial locomotion. Two experimental variables are used throughout the study: speed and body size. Mass-specific metabolic rates of running animals can be varied by about tenfold using either variable. This first paper considers metabolic energy consumed during terrestrial locomotion. New data relating rate of oxygen consumption and speed are reported for: eight species of wild and domestic artiodactyls; seven species of carnivores; four species of primates; and one species of rodent. These are combined with previously published data to formulate a new allometric equation relating mass-specific rates of oxygen consumed (VO2/Mb) during locomotion at a constant speed to speed and body mass (based on data from 62 avian and mammalian species): VO2/Mb = 0.533 Mb-0.316.vg + 0.300 Mb-0.303 where VO2/Mb has the units ml O2 s-1 kg-1; Mb is in kg; and vg is in m s-1. This equation can be expressed in terms of mass-specific rates of energy consumption (Emetab/Mb) using the energetic equivalent of 1 ml O2 = 20.1 J because the contribution of anaerobic glycolysis was negligible: Emetab/Mb = 10.7 Mb-0.316.vg + 6.03 Mb-0.303 where Emetab/Mb has the units watts/kg. This new relationship applies equally well to bipeds and quadrupeds and differs little from the allometric equation reported 12 years ago by Taylor, Schmid-Nielsen & Raab (1970). Ninety per cent of the values calculated from this genera equation for the diverse assortment of avian and mammalian species included in this regression fall within 25% of the observed values at the middle of the speed range where measurements were made. This agreement is impressive when one considers that mass-specific rates of oxygen consumption differed by more than 1400% over this size range of animals.