The neuropathologic conditions in 120 perinatal deaths attributed to fetal or newborn asphyxia were examined. Central nervous system necrosis was present in 16 of these deaths. The approximate time of asphyxial insult was established by determining the duration of the process, based on the findings of neuronal necrosis, macrophage response, or an astrocyte response, in conjunction with clinical data. The time of the asphyxial insult for the 16 perinatal deaths was as follows: antepartum fetal asphyxia, two cases; antepartum-intrapartum fetal asphyxia, five cases; intrapartum fetal asphyxia, four cases; and neonatal asphyxia, five cases. These observations indicate that an asphyxial insult may occur in the antepartum period, in the prodromal period of preterm labor, in the intrapartum period, and in the neonatal period. Five to ten percent of the asphyxial insults in each reproductive time period were initially sublethal, allowing necrosis of the brain of the fetus or newborn to develop.