Evidence suggests that motor skill training is accompanied by changes in motor cortical excitability in human subjects (Karni et al. 1995; Classen et al. 1998; Lotze et al. 2003). However, little is known about the effect of motor skill training on spinal neuronal circuits (Nadler et al. 2000, Hess et al. 2003). With local ethics committee approval we investigated the effect on the soleus H-reflex of a 30 min period of motor skill training involving the ankle muscles in 10 healthy human subjects. The subjects were instructed to make a cursor follow a series of target lines on a computer screen by performing voluntary ankle plantarflexion-dorsiflexion movements. H-max and H-max/M-max ratios were significantly depressed immediately after training and returned to baseline after 10 min. The slope of the H-reflex recruitment curve was also significantly lower immediately after training. No changes were observed in M-max after training. To elucidate the mechanisms underlying these H-reflex changes we measured the size of the long-latency (60 ms) depression of the soleus H-reflex evoked by peroneal nerve stimuation (D1 inhibition) and the size of the facilitation of the soleus H-reflex evoked by femoral nerve stimulation. Both measures are assumed to be influenced by changes in presynaptic inhibition of soleus Ia afferents. We observed a significant increase in the D1 inhibition and a decrease in the femoral nerve facilitation following the training with a return to baseline after 10 min. These findings suggest that increased presynaptic inhibition of soleus Ia afferents contribute to the depression of the H-reflex. In contrast, there were no changes in the short-latency (2-3 ms) inhibition of the soleus H-reflex evoked by peroneal stimulation suggesting that there were no changes in disynaptic reciprocal inhibition in relation to the training. The data demonstrate that motor skill training may change the transmission in spinal cord circuitries. Such changes may be of importance in the rehabilitation of motor disorders