To elucidate the origin of the early component of the trigeminal sensory evoked potential (TSEP) we reported in 1985, we recorded evoked potentials on the trigeminal root and the brain stem surface intracranially when suboccipital craniectomy was done for exploration of the C-P angle. Patients consisted of 4 who had idiopathic trigeminal neuralgia with microvascular decompression, 1 with glossopharyngeal neuralgia who also had microvascular decompression of the left glossopharyngeal nerve, and 5 patients with different tumors, specifically, Meckel's cave meningioma, two epidermoid cysts of the cerebello-pontine angle, an acoustic neurinoma and a trigeminal neurinoma. Among the tumor patients, all but the one with acoustic neurinoma had trigeminal neuralgia. So altogether, there were 8 patients with trigeminal neuralgia. Constant electrical current was applied to the lower and upper lip for stimulation of the second and third division, respectively. In some cases the electrical current was applied percutaneously to the infraorbital nerve for the stimulation of the second division. The supraorbital nerve was stimulated percutaneously for stimulation of the first division. To record and analyze the potentials, Evomatic 8000 (Dantec) was used. Analysis time was from 10 to 100 msec and the stimulation was repeated 200 times. Trigeminal compound action potential (TCAP) was recorded on the trigeminal nerve just distal to the entrance point to the pons. For the recording TCAP, a gold ball electrode of 1 mm diameter was used with indifferent needle electrode in the neck muscle. In some cases a digital filter was applied to the recorded TCAP. The TCAP evoked by stimulation of the second division had 2 to 8 positive potentials in 8 patients.(ABSTRACT TRUNCATED AT 250 WORDS)