Transcranial magnetic stimulation (TMS) was employed before and after a single dose of gabapentin to evaluate how this drug affects the activity of excitatory and inhibitory circuits within the motor cortex. Eleven healthy volunteers were studied. For the evaluation of cortical excitability, the following parameters were taken into account: resting and active motor threshold (RMT, AMT); cortical silent period (CSP); and intracortical inhibition (ICI) and facilitation (ICF). Peripheral silent period (PSP) was also detected. All parameters were measured before and 3 and 24 hours after 800 mg gabapentin was administered in a single oral dose. Gabapentin deepened the ICI and suppressed the ICF at 3 h but not at 5 h after dosing. We conclude that, in the normal human brain, gabapentin may act on intracortical excitability by shifting the balance towards less excitation and more inhibition.