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 + theta f, where P = power output, W' = slope of regression, and theta f = 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 relationship for runs of 2-12 min duration conformed to a similar hyperbolic function as that described for cycle ergometry. The two parameters W' and theta f might provide valuable indices of physical performance potential, which can be used to monitor training responses in competitive runners.