Stimulation of cutaneous afferent fibres in the sural nerve and muscle afferent fibres in the gastrocnemius-soleus nerve at a strength that excites C axons produces a delayed and long-lasting burst of activity in posterior biceps femoris/semitendinosus flexor motoneurones. Following a 20 s stimulation at 1 Hz to the sural nerve the flexor motoneurones continue to fire for 20 s while a similar stimulus to gastrocnemius-soleus nerve results in an after-discharge lasting three times longer. Using stimuli to the sural and gastrocnemius-soleus nerves as conditioning stimuli (20 s, 1 Hz) changes in the excitability of the flexor reflex were measured by recording the discharge evoked by a test sural nerve stimulus or by a standard pinch to the ipsilateral and contralateral toes. Prior to any conditioning stimulus the flexor reflex remained stable for prolonged periods. Conditioning stimuli at strengths that activated large myelinated afferent fibres only, or large and small myelinated afferent fibres, failed to produce more than a very transient alteration in the reflex excitability. Conditioning stimuli at C-fibre strength to the sural nerve produced a marked increase in the excitability of the reflex for 10 min. C-fibre strength gastrocnemius-soleus nerve conditioning stimuli resulted in a similar increase in excitability followed by a second phase of facilitation peaking at 20-30 min and lasting for up to 90 min. The afferent barrage initiated by cutting the sural and gastrocnemius-soleus nerves resulted in similar patterns of reflex excitability increases with the muscle nerve resulting in a more prolonged effect than the cutaneous nerve. The results show that a brief C-afferent fibre input into the spinal cord can produce a prolonged increase in the excitability of the flexion reflex and that muscle C-afferent fibres evoke longer-lasting changes than cutaneous C fibres. The differences in the time course of the post-conditioning effects may be related to the well-described differences in the sensory consequences of injury to skin versus deep tissue.