Cerebrospinal fluid (CSF) derived from six patients with polyradiculoneuritis (Guillain-Barré syndrome, GBS) treated by liquorpheresis was injected into rat sciatic nerve. By measuring spinal evoked potentials after stimulation of the tibial nerve, we observed slowing or dispersion of nerve conduction in those cases where the CSF had been taken before liquorpheresis. CSF of the same patient, sampled after liquorpheresis, showed minor effects only. Impairment of nerve conduction was seen between 5 and 20 min after injection, normal function being restored on the third day. These results suggest that liquorpheresis eliminates blocking factors from the CSF of patients with GBS. We postulate this as the effect by which liquorpheresis improves neurological symptoms in Guillain-Barré syndrome.