Nineteen patients with unilateral supratentorial mass lesion and without any evident clinical signs of transtentorial herniation were studied with Computed Tomography (CT), brain-stem evoked potentials (BAEPs) and central conduction time (CCT) of short latency somatosensory evoked potentials (SEPs). Sixteen had tumours, two had intracranial haematoma and one had chronic subdural haematoma. CT detected the initial signs of transtentorial herniation in every case. Preoperative I-V interpeak latency (IPL) was significantly (M + 2SD) prolonged in 26% of cases on the lesion side and in 21% of cases on the opposite side. The mean I-V IPL was significantly prolonged both on the lesion side and the opposite side (P less than 0.01, P less than 0.02, respectively). Suppression of Wave V (M-2SD) was seen only in two cases, however, the mean amplitude of Wave V was significantly decreased both on the lesion side and on the opposite side (P less than 0.001, P less than 0.01, respectively). CCT of SEPs was significantly (M + 2SD) prolonged in 33% of cases on the lesion side and in only 13% on the opposite side. The mean CCT was, however, significantly prolonged both on the lesion and on the opposite side (P less than 0.001, P less than 0.02, respectively). Postoperative I-V IPL was significantly prolonged in only 11% of cases while the mean I-V IPL was still significantly prolonged (P less than 0.01) and the mean amplitude of Wave V was still suppressed (P less than 0.001) on the lesion side.(ABSTRACT TRUNCATED AT 250 WORDS)