To determine the effects of cardiopulmonary bypass with profound hypothermic circulatory arrest (PHCA) on anterior fontanel pressure (AFP) and visual evoked potentials (VEPs), 21 neonates and infants undergoing cardiopulmonary bypass (CPB) with PHCA for surgical correction of congenital heart defects were studied. Mean (+/- SD) minimum nasopharyngeal, esophageal, and rectal temperatures of 16.4 +/- 2.2, 11.2 +/- 2.7, and 17.7 +/- 1.9 degrees C, respectively, were achieved for a mean duration of PHCA of 51.6 +/- 18.7 min. AFP increased significantly above pre-CPB values for the first 21.7 +/- 8.1 min of rewarming. The duration of this increase in AFP was related logarithmically and directly to the product of the nasopharyngeal temperature (NPT) at the end of PHCA and the duration of PHCA (r2 = 0.82, P less than 0.0001). Nineteen of these patients had simultaneous monitoring of VEPs. The latency of both the N70 and P100 components of the VEPs increased as temperature decreased. The cerebral perfusion pressure was linearly and inversely related to the AFP (r2 = 0.72, P less than 0.01). The VEPs disappeared as a nasopharyngeal temperature (NPT) of 18.9 +/- 2.8 degrees C and reappeared after 21.9 +/- 8.8 min post-PHCA at an NPT of 32.8 +/- 1.4 degrees C. There was no significant difference between duration of increased AFP (20.9 +/- 8.1 min) and the duration of absence of VEPs during the post-PHCA period. The duration of increased AFP correlated linearly and directly with the duration of absence of VEPs (r2 = 0.84, P less than 0.005). These data demonstrate that transient neurophysiologic dysfunction occurs after PHCA. This dysfunction is related to the duration of elevation of the AFP and cannot be explained solely by a temperature effect.