The consequences of perinatal asphyxia on the rat brain were studied 80 min to 8 days after birth with hematoxylin-eosin and in situ DNA double-strand-breaks labeling histochemistry. Asphyxia was induced by immersing fetus-containing uterus horns, removed from ready-to-deliver Sprague-Dawley rats, in a water bath at 37 degrees C for various time periods (0-22 min). Spontaneous- and cesarean-delivered pups were used as controls. Perinatal asphyxia led to a decrease in the rate of survival, depending upon the length of the insult. No gross morphological changes could be seen in the brain of either control or asphyctic pups at any of the studied time points after delivery. However, in all groups, nuclear chromatin fragmentation, corresponding to in situ detection of DNA fragmentation, was observed at different stages. Nuclear fragmentation in control pups showed a specific distribution that appeared to be related to brain maturation, thus indicating programmed cell death. A progressive and delayed increase in nuclear fragmentation was found in asphyctic pups, which was dependent upon the length of the perinatal insult. The most evident effect was seen in frontal cortex, striatum, and cerebellum at postnatal day 8, although changes were also found in ventral-posterior thalamus, at days 1 and 2. Thus, nuclear chromatin fragmentation in asphyctic pups indicates a delayed post-asphyctic neuronal death. The absence of signs of inflammation or necrosis suggests that delayed neuronal cell death following perinatal asphyxia is an active, apoptosis-like phenomenon.