Patients with central spastic paresis and equipped with peroneal stimulators sometimes experience an improvement in their gait, even when the stimulator has been switched off. The object of the present investigation was to reach a better understanding of the mechanisms operating in such cases. 7 patients were investigated on repeated occasions. It was found that some of these patients got a clear increase in isometric strength of foot dorsiflexion following 10 min of peroneal stimulation. In other cases the increase was only slight. There was an inverse relation between the increase and the strength before stimulation. The increase of strength was due, at least partly, to an increased ability to activate the foot dorsiflectors, since there was a simultaneous increase in the EMG from the anterior tibial muscle. Evidence was also obtained suggesting that the increase in strength involved not only foot dorsiflexion but also plantarflexion of the foot end extension of the knee. Following peroneal stimulation there was also a decrease of the achilles reflex and in some cases possibly also the patellar reflex. There was an inverse relation between the decrease in the achilles reflex and its strength before stimulation. It is probable that the changes in voluntary strength and reflex activity reflect conditions which can be of importance in explaining the gait improvement which is sometimes observed in patients equippped with peroneal stimulators.