To test the effect of motor imagery as compared to voluntary muscle contraction on the excitability of the anterior horn cells, we studied F-wave persistence and amplitude, monitoring the force by a transducer. F-waves were recorded from the first dorsal interosseus muscle at rest and under a battery of motor tasks which comprised mental imagery and mild to moderate abduction of the index finger in 10 increments up to 30% of the maximal force. During each step lasting for 20s, 20 stimuli were applied to the ulnar nerve at the wrist at the rate of one per second. In 12 healthy subjects, F-wave persistence (mean±SD) significantly (p<0.05) increased from 32.5±11.9% at rest to 58.3±15.2% during motor imagery and to 87–91% during voluntary contraction. Similarly, F-wave amplitude significantly (p<0.05) increased from 41.3±77.1μV at rest to 91.2±142.7μV during motor imagery and to 183–274μV during voluntary contraction. In contrast, F-wave latency changed little regardless of motor tasks. Motor imagery, though to a lesser degree than muscle contraction, effectively enhances the excitability of the anterior horn cells, thereby increasing F-wave persistence and amplitude above the baseline.