The factors that influence the functional integrity of the central nervous system during clinical procedures involving profoundly hypothermic circulatory arrest (PHCA) have not been objectively evaluated. Intraoperative monitoring of somatosensory evoked potentials (SEPs) was performed in nine infants undergoing PHCA during repair of congenital cardiac anomalies to investigate the short-term effects of this intervention on neurophysiologic function. Latency prolongation of the primary cortical (N18,P22) and cervical spinal cord (N13) responses, reflecting slowing of neural transmission with hypothermia, occurred as a power function of decreasing systemic temperature (p less than .01). The cortical evoked response disappeared during profound hypothermia (less than 18 degrees C), remaining absent throughout the period of circulatory arrest and for a variable period of time after reperfusion. Regression analysis indicated that the time required for the recovery of the cortical evoked response on reperfusion was a linear function of the time-temperature integral of the arrest period (p less than .001) and the pH at the onset of circulatory arrest (p less than .001). Neurologic complications occurred in three patients and included cortical blindness (n = 2) and a generalized seizure disorder (n = 1). Visual dysfunction was not reflected in the intraoperative SEP recordings, whereas prolonged delay of SEP recovery, indicative of global central nervous system injury, was observed in the patient who experienced seizures after the surgery. This preliminary experience with SEP monitoring during PHCA suggests a role for this modality in determining the short-term effects of this procedure on neurophysiologic function. The recovery characteristics of somatosensory neural transmission appear to be modulated by the duration of, and temperature and pH maintained during, the arrest period.