The purpose of this study was to find out whether or not the lions' reliance on social hunting is correlated with physiological constraints which limit their speed and endurance. Oxygen consumption of two young male lions was measured as a function of running speed, first when they weighed 28-35 kg and again after they had approximately doubled their weight (50-57 kg). Oxygen consumption increased linearly with speed between 2.0 and 8.5 km·h⁻¹, but at a rate more than three times the rate predicted for quadrupedal mammals. There was no difference between the weight-specific oxygen consumption of the lions as they doubled their weight. The observed slope for the relationship between oxygen consumption and speed was 0.36 ml O₂ (g·km)⁻¹ versus a predicted value of 0.11 ml O₂ (g·km)⁻¹. The observed y-intercept was -0.09 ml O₂ (g·h)⁻¹ versus a predicted value of 0.39 ml O₂ (g·h)⁻¹. The lions' maximum aerobic capacity appeared to be about 3 ml O₂ (g·h)⁻¹, a value about 13 times their predicted resting rate. This was reached at low speeds where the lions traveled at a slow trot because oxygen consumption increased so rapidly with increasing speed. The same maximum value was obtained on a level and on an inclined treadmill. Lions change from a trot to a gallop at a lower stride frequency than would be predicted. We are not able to account for the lions' unusually high energy cost for locomotion. Neither their limb morphology nor their immaturity offers a likely explanation. Our results help explain the lions' reliance on social hunting on physiological grounds.