SUMMARY Percutaneous electrical stimulation of the trigeminal root was performed in 18 subjects undergoing surgery for idiopathic trigeminal neuralgia or implantation of electrodes into Meckel's cave for recording of limbic epileptic activity. All subjects had normal trigeminal reflexes and evoked potentials. Sensory action potentials were recorded antidromically from the supraorbital (VI), infraorbital (V2) and mental (V3) nerves. In the awake subject, sensory potentials were usually followed by myogenic artifacts due to direct activation of masticatory muscles or reflex activation of facial muscles. In the anaesthetised and curarised subject, sensory potentials from the three nerves showed 1 4-2-2 ms onset latency, 1-9-2-7 ms peak latency and 17-29 pV amplitude. Sensory conduction velocity was computed at the onset latency (maximum CV) and 'at the peak latency (peak CV). On average, maximum and peak CV were 52 and 39 m/s for VI, 54 and 42 m/s for V2 and 54 and 44 m/s for V3. There was no apparent difference in CV between subjects with trigeminal neuralgia and those with epilepsy. A significant inverse correlation was found between CV and age, the overall maximum CV declining from 59 m/s (16 years) to 49 m/s (73 years). This range of CV is compatible both with histometric data and previous electrophysiological findings on trigeminal nerve conduction. Intraoperative intracranial stimulation is also proposed as a method of monitoring trigeminal function under general anaesthesia.The cutaneous branches of the trigeminal nerve remain in the depth of craniofacial structures for most of their course, and emerge on the surface of the face in the proximity of their target skin territory, dividing into thin, short, terminal fascicles. This leaves little room for stimulation and recording of nerve fibres in intact man, hence the absence of normative data on trigeminal nerve conduction.The available electroneurographic data have been achieved by intraoperative recordings, either by recording from' 2 or by stimulation of34 the ganglion and root, in subjects undergoing surgery for trigeminal neuralgia. This paper describes the sensory action potentials (SAPs) evoked in the supraorbital, infraorbital and mental nerves, by direct electrical stimulation of the trigeminal root, and the sensory conduction velocity