The power-endurance time curve described for cycle ergometry has been examined with respect to high velocity treadmill running to exhaustion. On a cycle ergometer, a hyperbolic function has been described that can be linearized'by expressing power relative to the inverse of time to exhaustion. The mathematical relationship of this linear function is P = W'/t + θf, where P = power output, W' = slope of regression, and θf = fatigue threshold (intercept of power output). Six cross-country runners took part as subjects. Each ran to exhaustion with the treadmill at 6 different velocities between 19.2 and 22.4 km/h. The linear regressions fitted to velocity versus 1/time had correlation coefficients between r = 0.979 and 0.997. It was concluded that the treadmill velocity-endurance time realtionship for runs of 2-12 min duration conformed to a similar hyperbolic function as that described for cycle ergometry. The two parameters W' and θf might provide valuable indices of physical performance potential, which can be used to monitor training responses in competitive runners.