Kinesthetic motor imagery (MI) is the mental simulation of a movement with its sensory consequences but without its concomitant execution. While the effect of MI practice on cortical areas is well known, its influence on spinal circuitry remains unclear. Here, we assessed plastic changes in spinal structures following an acute MI practice. Thirteen young healthy participants accomplished two experimental sessions: a 20min MI training consisting of four blocks of 25 imagined maximal isometric plantar flexions, and a 20min rest (control session). The level of spinal presynaptic inhibition was assessed by conditioning the triceps surae spinal H-reflex with two methods: (i) the stimulation of the common peroneal nerve that induced D1 presynaptic inhibition (H-PSI response), and (ii) the stimulation of the femoral nerve that induced heteronymous Ia facilitation (H-FAC response). We then compared the effects of MI on unconditioned (H-TEST) and conditioned (H-PSI and H-FAC) responses before, immediately after and 10min after the 20min session. After resting for 20min, no changes were observed on the recorded parameters. After MI practice, the amplitude of rest H-TEST was unchanged, while H-PSI and H-FAC significantly increased, showing a reduction of presynaptic inhibition with no impact on the afferent-motoneuronal synapse. The current results revealed the acute effect of MI practice on baseline spinal presynaptic inhibition, increasing the sensitivity of the spinal circuitry to MI. These findings will help in understanding the mechanisms of neural plasticity following chronic practice.