The distribution of somatosensory evoked potentials (SEPs) recorded from the brain-stem surface was studied to investigate their generator sources in 14 patients during surgical exploration of the posterior fossa. Two distinct SEPs of different morphologies and electrical orientation were obtained by median nerve stimulation. A small positive-large negative-late prolonged positive wave was recorded from the cuneate nucleus and its vicinity. There was a phase-reversal between the cuneate nucleus and the ventral surface of the medulla, depicting a dipole for dorso-ventral organization. From the pons and midbrain, triphasic waves with predominant negativity were obtained. This type of SEP had identical wave forms between the dorsal, lateral and ventral surface of the pons and midbrain. It showed an increase in negative peak latency as the recording sites moved rostrally, suggesting an ascending axial orientation. In a patient with pontine hemorrhage, the killed end potential, a large monophasic positive potential was obtained from the lesion. This potential occurs when an impulse approaches but never passes beyond the recording electrode. Therefore, the triphasic SEP from the pons and midbrain reflects an axonal potential generated in the medial lemniscal pathway.