The strict division between motor and somatosensory systems might be less distinct than previously thought. Many brain mapping studies have described changes of somatosensory cortex (S-I) after the execution of a motor task, which supports the idea of a profound interconnectedness in the sensorimotor system. Here we report experiments in which we investigated by means of somatosensory evoked potentials (SSEPs) mapping the reorganizational capacities in primary somatosensory cortex before and after a Hebbian repetitive co-contraction task of the thumb and arm. We investigated the susceptibility of S-I plasticity to the pharmacological modulation of the GABA-neurotransmitter system by application of the GABA(A) agonist lorazepam. We found that repetitive training induced stable motor learning characterized by a significant improvement of performance. The time differences between the onset of contraction of the deltoid muscle and the abductor pollicis brevis were progressively shortened. The process of motor learning was accompanied by plastic changes in the primary somatosensory cortex as indicated by a significant increase in the dipole strength and a significant shift of the median nerve dipole on the hemisphere contralateral to the exercised side. Moreover, the individual shifts of median nerve dipole location were correlated with the individual improvement in motor performance. After administration of lorazepam, motor learning was significantly suppressed. The behavioural effect was accompanied by an abolition of the N20 dipole shift and an unchanged dipole strength. The results imply that motor learning leads to a profound reorganization in S-I which is subject to pharmacological suppression with the GABA agonist lorazepam.