Studies in animal models and humans indicate that chronic nicotine intake influences neuronal excitability, resulting in functional and structural CNS changes. The aim of the present study was to explore human primary motor cortex (M1) excitability with transcranial magnetic stimulation (TMS) in chronic smokers. A total of 44 right-handed volunteers, aged 20-30 years, participated in the study. Chronic smokers were compared with age- and sex-matched healthy nonsmokers. We tested cortical excitability with single- and paired-pulse TMS to the left M1 and short-latency afferent inhibition (SAI) by combining median nerve stimulation and motor cortex TMS. Compared with nonsmoking controls, chronic smokers showed a significantly larger amount of SAI, which is thought to depend upon the activity of cholinergic inhibitory circuits produced by somatosensory inputs. Moreover, TMS-evoked inhibitory cortical silent periods were prolonged, whereas paired-pulse intracortical facilitation and motor-evoked potentials during moderate contraction were reduced. The results suggest that chronic nicotine intake may not only strengthen cholinergic inhibitory circuits, but could also be associated with enhanced inhibitory and reduced facilitatory mechanism of specific neuronal circuits in motor cortex. These changes may form a physiological basis for neurobiological and behavioral variations associated with chronic smoking.