The study was performed on a group of 17 patients with spastic paraparesis: 12 with hereditary spastic paraparesis, 3 with cord compression and 2 with complete spinal transection. 10 healthy volunteers acted as controls. Recurrent inhibition of the soleus alpha-motoneurones was estimated at rest and during voluntary contraction of triceps surae. At rest, there was evidence for a substantial decrease in the excitability of Renshaw cells in 9 out of the 12 patients with hereditary spastic paraparesis; this was also observed in the 2 patients with complete spinal transection, while the 3 patients with cord compression exhibited a normal Renshaw cell activity. In 3 out of 4 patients with hereditary spastic paraparesis, the changes in Renshaw cell excitability expected to occur during voluntary contraction were not found, whereas in one patient with hereditary spastic paraparesis and one with spastic paraparesis due to cord compression recurrent inhibition was normally influenced by the motor command. Our results indicate that recurrent inhibition is likely to be differently affected according to the type and/or localization of the lesion. It is also suggested that the central nervous system might control the excitability of Renshaw cells at rest and during voluntary contraction via partly separate pathways. The role of recurrent inhibition in spasticity is discussed.