We constructed a force treadmill to measure the vertical, horizontal and lateral components of the ground-reaction forces (F_z, F_y, F_x, respectively) and the ground-reaction force moments (M_z, M_y, M_x), respectively exerted by walking and running humans. The chassis of a custom-built, lightweight (90 kg), mechanically stiff treadmill was supported along its length by a large commercial force platform. The natural frequencies of vibration were >178 Hz for F_z and >87 Hz for F_y, i.e., well above the signal content of these ground-reaction forces. Mechanical tests and comparisons with data obtained from a force platform runway indicated that the force treadmill recorded F_z, F_y, M_x and M_y ground-reaction forces and moments accurately. Although the lowest natural frequency of vibration was 88 Hz for F_x, the signal-to-noise ratios for F_x and Mz were unacceptable. This device greatly decreases the time and laboratory space required for locomotion experiments and clinical evaluations. The modular design allows for independent use of both treadmill and force platform.