Central motor conduction (CMC) studies were carried out in 25 patients with hereditary spastic paraplegia (HSP). Responses evoked in the lower limbs by transcranial magnetic stimulation of the motor cortex were bilaterally absent in 33% of the patients and, when recordable, were delayed in 75% of cases. Responses in the upper limbs were mostly normal except for those from the five members of one family, which were considerably delayed. There was no correlation between CMC parameters and age, duration of disease or upper limb hyperreflexia. CMC time to the tibialis anterior correlated with disability in patients with juvenile-onset HSP. It is concluded that CMC studies are not useful in detecting subclinical lesions in hereditary spastic paraplegia but may be of value in identifying subgroups of the disease.