Somatosensory evoked potentials recorded over Erb's point and the cervical spine (at C2 and C7) were studied in a series of children undergoing cardiopulmonary bypass surgery with hypothermia alone (n = 15) or profound hypothermia and complete circulatory arrest (n = 15). A bifid response was recorded at normothermia or mild degrees of hypothermia at both C7 (N12a, N13a) and C2 (N12b, N13b). The differential responses of these components to profound hypothermia and ischaemia suggest that N12a and N12b represent components of the same dorsal root/dorsal column travelling wave, while N13a and N13b reflect postsynaptic activities which are thought to be generated at the dorsal horn and cuneate nucleus, respectively.