Mitochondrial disorders are characterised by protean neurological manifestations including peripheral neuropathy. The neuropathy is typically an axonal process, with a proposed mechanism being degradation of the membrane potential of the axon due to dysfunction of energy-dependent Na(+)/K(+) ATPase pump. To address this issue, we studied the excitability of motor axons in the median nerve in 16 individuals identified with gene testing or muscle biopsy. Twelve patients had clinical or nerve conduction abnormalities. Strength--duration time constant, threshold electrotonus, current--threshold relationship and recovery cycle were identical to control values. These findings imply that an alternative mechanism to a change in membrane potential is responsible for neuronal injury in these patients.