Neural cuffs, implanted around various hindlimb nerves (sural, common peroneal, posterior tibial), were used to deliver brief stimulus trains to unrestrained cats walking on a treadmill. The resulting perturbations of the step cycle were evaluated by analyzing the EMG bursts recorded from the ankle extensors and by high speed cinematography. It was found that relatively weak stimulation (1.4 to 2 X T) of the posterior tibial nerve was very effective in eliciting a prolongation of the flexion phase provided the stimuli were applied just prior to the expected onset of the ankle extensor EMG burst. This ipsilateral hyperflexion was correlated with a prolongation of the contralateral extension. The same stimuli applied during stance usually evoked a yielding of the stimulated leg and a prolongation of the ongoing contralateral stance. In addition to these flexor and extensor reflex effects, it was found that low threshold stimulation of the sural and common peroneal nerves resulted in a powerful reflex activation of the ankle extensors. In contrast, stimulation of the gastrocnemius-soleus nerve (a muscle nerve) produced no discernible behavioral effects, even for stimuli at 3 X T, indicating that the observed reflexes are probably mediated by cutaneous afferents. The results were largely confirmed in experiments using the same cuffs implanted in spontaneously walking premammilliary cats.