1. The effects of stimulating hindlimb extensor nerves (100-200 ms trains, 100 Hz, < or = 2 times threshold) during the flexor and extensor phases of the locomotor step cycle were analysed in the decerebrate, paralysed cat during fictive locomotion evoked by stimulation of the mesencephalic locomotor region. 2. Stimulation during extension of either the medial gastrocnemius (MG), lateral gastrocnemius-soleus (LGS) or plantaris (Pl) nerves was equally effective in increasing the duration and amplitude of electroneurogram (ENG) activity recorded in ipsilateral ankle, knee and hip extensor nerves. Enhancement of extensor ENG activity could be evoked with near threshold stimulation intensity and appeared within 10-40 ms of the onset of ankle extensor nerve stimulation. Stimulation of anterior biceps during extension occasionally evoked a modest increase in the duration of activity of hip, knee and ankle extensors. Stimulation of quadriceps during extension enhanced the activity of proximal extensors and soleus, but inhibited other ankle extensors. 3. Selective activation of ankle extensor Ia spindle afferents by muscle stretch also enhanced ipsilateral extension. It is argued that both muscle spindle and tendon organ afferents can contribute to the increase in extensor nerve activity evoked by group I stimulation intensity during fictive locomotion. 4. During flexion, stimulation of either the MG, Pl or LGS nerves at group I strength terminated on-going activity in ipsilateral flexors and initiated a burst of activity in ipsilateral hip, knee and ankle extensors, i.e. reset the step cycle to extension. 5. Low strength stimulation of the mixed muscle and cutaneous nerve innervating the plantar aspect of the foot produced extension enhancement and resetting similar to that evoked by group I muscle afferent stimulation. Stimulation of the cutaneous nerve supplying the dorsal aspect of the foot during extension enhanced extensor activity, and during flexion, enhanced the activity of flexors. 6. The effects reported here during fictive locomotion may also occur during overground locomotion with natural activation of group I muscle spindle and tendon organ afferents. Extensor spindle and tendon organ afferents may thus serve as an excitatory reflex system helping to shape the amplitude, duration and timing of ipsilateral extensor activity. Increased or unexpected activation of group I ankle extensor afferents or plantar foot afferents during locomotion could also compensate for increased loading of the limb.