Recurrent inhibition via Renshaw cells provides a mechanism by which spinal and supraspinal centers exert control over movement. The conditioned H-reflex technique of Pierrot-Deseilligny and Bussel permits noninvasive assessment of recurrent inhibitory pathways. We employed this technique to investigate changes in Renshaw cell activity due to nicotine (a potent CNS cholinergic agonist that excites Renshaw cells in animals) contained in inhaled tobacco smoke. In 10 normal subjects, cigarette smoking caused a large, rapid drop in the conditioned H-re-sponse amplitude, implying increased activation of Renshaw cells. The time course of the change in conditioned H-re-sponse amplitude closely approximated the known pharmacokinetics of inhaled nicotine. Nicotine administered via chewing gum had a much slower and less dramatic effect, probably due to the slower rise in blood levels with this mode of administration. Increased activity in Renshaw cells may contribute to spasticity in spinal cord-injured patients, raising the possibility that cigarette smoking could cause further increases in tone in such patients.