There are at least seven clinically indistinguishable but genetically different types of autosomal dominant pure spastic paraplegia (ADPSP). In this study we investigated electrophysiological characteristics in patients with ADPSP linked to chromosome 2p (SPG4). Twelve patients from six different families with ADPSP linked to chromosome 2p and 15 control persons were included. Electromyography (EMG), motor and sensory nerve conduction, and motor evoked potentials using single and paired transcranial magnetic stimulation (PTMS) was performed. From the peripheral nervous system we found signs of motor and sensory axonal neuropathy. Motor evoked potentials disclosed greatly reduced corticospinal tract conduction velocity and amplitude of evoked potentials to the lower extremities indicating that the very marked spasticity predominantly seems to rely on dysfunction of the fast conducting axons of the pyramidal tract. PTMS showed an increased intracortical facilitation (ICF), which may reflect an impaired function of gamma-aminobutyric acid (GABA)-controlled interneuronal circuits in the motor cortex, alternatively an increased glutamatergic transmission or a compensatory recruitment of a larger number of neurones with corticospinal projections.