Rates of oxygen consumption were measured for two bipedal runners (two species of quail) and two quadrupedal runners (two small species of rodents), with average body masses that ranged from 0.035 to 0.217 kg, trained to run on a treadmill set to horizontal and then to a 10 degrees incline. Rates of oxygen consumption increased linearly with speed for all four species and the rates of increase were significantly higher (P < or = 0.05) for all four species when the animals were run on an incline than when they were run on a horizontal. The estimated metabolic energy cost to lift 1 kg mass 1 m vertically was similar for bipeds and quadrupeds of similar body mass and inversely related to body mass for both running styles. When the data for the animals used in the present study are combined with similar data for adult animals from the literature, the results show that the metabolic energy efficiencies of locomotion, estimated from the cost of vertical work, are the same for bipedal and quadrupedal runners. In both groups, the metabolic energy efficiency of locomotion is directly related to body mass for animals smaller than 1 kg body mass.