The soleus H-reflex amplitude is deeply modulated during locomotion in humans (Capaday and Stein, 1986). Moreover, at a constant stimulus intensity, the slope of the relationship between the amplitude of the soleus H-reflex and the background electromyogram (EMG) changes with different locomotor tasks (Capaday and Stein, 1987a). Two further aspects are studied here. First, we recorded the reflex during overlapping speeds of walking (2.0–7.5 km/hr) and running (5–9 km/hr) to determine whether the speed, the motor output, or the form of locomotion was most important in setting the slope of this relationship between H-reflex and background EMG. Second, we determined the time course of change in the H-reflex amplitude and the possible site of action for the reflex depression during the transition from standing to walking. The primary determinant of the slope was found to be the form of locomotion. The differences between running and walking could not be explained entirely by either movement speed or motor output. For walking, the slope varied inversely with the speed and the motor output of locomotion. This compensation in slope as a function of motor output may prevent saturation of the motoneuron pool. The appropriate reflex amplitudes for a particular locomotor pattern are activated rapidly and completely within a reaction time, and simultaneously with the activation of muscle activity for the initiation of walking. Mechanisms for the rapid change seen during the initiation of locomotion most likely act presynaptically on the muscle spindle afferents. The time course and magnitude of this change are correlated with the activity of the tibialis anterior muscle.