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This article deals with electrophysio­logical characteristics of the long sensory tracts and the interneuronic system of the spinal cord in patients operated on the spinal cord. Spinal somatosensory evoked potentials (SSEP) were recorded using the subpial and the epidural technique during intraoperative neuromonitoring, which is obligatory in all operations on the spinal cord performed at the Department of Neuro­surgery, Ljubljana. The median, tibial and sural nerves were stimulated. Somatosensory evoked potentials of the long sensory tracts and of the spinal interneuronic system consist of different elements. Changes in these elements indicate damage to the parts of the spinal cord that gen­erate them. The most stable element of the conducted SSEP are the initial negative waves, which originate in the spinocerebellar tract. Deterioration in the shape of the negative high amplitude potentials (NHP) is the most sensitive indicator of damage to the somatosensory sys­tem of the spinal cord. Their disappearance or absence indicate severe damage to the dorsal columns. The negative low amplitude poten­tials (NLP) are the most sensitive element of the conducted SSEP Their absence indicates dam­age to the somatosensory system of the spinal cord. Wich can not be precisely localized. The N11 and N14 waves are the most stable ele­ments of the cevical and lumbo-sacral interneu­ronic SSEP Since they remain unchanged even in cases of severe damage to the dorsal columns they most probably originate in the dorsal root entry zone. The N13 and N17 waves are the most frequently changed elements of the cervical and lumbo-sacral interneuronic SSEP Changes of these two waves suggest the damage to the spinal cord gray matter. High frequency waves seen on the slow negative wave of the interneu­ronic SSEP or appearing in the place of the absent N13 wave most probably reflect the activity of the somatosensory long tracts. Clinical evidence of somatosensory lesions is associated with the changes of the SSEP elements. They enable us to localize the anatomical site of the lesion, which is of a great help to the operating neurosurgeon.