RECENT studies from this laboratory1have shown that there is a functionally significant relationship between the size of individual motor neurons and their susceptibility to discharge by physiological stimuli. The smaller the cell, the more readily it is discharged by all normal stimuli. This sizeprinciple has been demonstrated in a variety of experiments in which discharge of motor neurons has been brought about by stimulation of the motor cortex, basal ganglia, cerebellum, or brainstem,2or by evoking stretch reflexes, flexor reflexes, or crossed reflexes.1,3 The present study was begun after recordings of muscle action potentials in stretch reflexes revealed an orderly recruitment of units of increasing amplitude, which was strikingly similar to that seen in records from ventral root filaments. There have been several reports4-7in which it was noted that the amplitude of individual electromyographic (EMG) potentials increased with the total amount of excitation.