The effect of a single dose of oral carbamazepine on cortical facilitatory and inhibitory phenomena was investigated in 13 healthy human subjects by focal transcranial magnetic stimulation. Paired stimulation was performed via a figure-of-eight shaped magnetic coil using two Magstim-200 stimulators and a Bistim-interface at interstimulus intervals of 3, 10, and 17 ms. In addition, the silent period evoked by single focal transcranial stimuli during sustained voluntary muscle contraction was investigated without and with carbamazepine. Recordings of compound muscle action potentials (CMAP) were taken from the left first dorsal interosseus muscle. Carbamazepine significantly reduced the facilitatory effect of conditioning stimuli of 85% of motor threshold at an interstimulus interval of 10 ms on the CMAP-amplitude from 162% to 127%, whereas under all other conditions no significant depression of CMAP-amplitudes occurred. This effect is discussed in the context of carbamazepine's use-dependent inhibition of neuronal high-frequency discharges. The mean relative duration of the silent period was longer with carbamazepine at all 6 stimulus intensities investigated, the absolute effect being very low in relation to the interindividual variability of silent period duration. The study demonstrates the applicability of transcranial magnetic stimulation as an in vivo method in the assessment of drug effects on cortical facilitatory as well as inhibitory phenomena.