Electrophysiological studies have shown that 1-Hz repetitive transcranial magnetic stimulation (rTMS) of the primary motor area (M1) can produce a local decrease in excitability. Functional imaging data suggest that this change may be bilateral. In normal subjects, we measured motor evoked potential (MEP) amplitude at a series of stimulation intensities in the contralateral M1 before and after 15 min of active or sham rTMS at just above the MEP threshold. The slope of the curve relating MEP amplitude and stimulation intensity was decreased in the unstimulated hemisphere by active but not sham rTMS. This demonstrates that rTMS can condition cortical excitability at a distance of one or more synapses and suggest that decreased excitability to TMS is a correlate of decreased blood flow and metabolism.