Direct electrical stimulation of the intracranial portion of the trigeminal nerve was performed in 23 subjects undergoing retrogasserian thermocoagulation for the treatment of idiopathic trigeminal neuralgia. In 16 subjects, who were having the operation for the first time, neurological examination was normal, as was neurophysiological testing of trigeminal function. Seven subjects were being operated for the second time, owing to a recurrence of symptoms. In all the subjects being operated for the first time, direct motor responses were obtained from ipsilateral temporalis, masseter and anterior belly of the digastric. The motor conduction velocity was equal for the fibres directed to all three muscles. This was estimated to be 54m/s in the masseteric nerve and 55-68 m/s in the intracranial portion of the trigeminal nerve. Patients who had undergone previous thermocoagulation had a considerably slower conduction velocity. It is supposed that myelin sheaths had been damaged at the first operation.