We investigated the adverse effect of hypocapnia on the neonatal rabbit brain. Two-week-old Japanese white rabbits were assigned to three groups, hyperventilation (H group), ischemia (I group), or hypocapnia with ischemia (HI group) and then subjected for 1.5 h with simultaneous measurement of the mean arterial blood pressure (MABP) and intracranial Hb concentration changes. Marked reductions of PaCO2 and MABP were induced in the hyperventilation-loaded groups and the ischemia-loaded groups, respectively. The intracranial oxyhemoglobin and total Hb concentrations decreased slightly in the H group and markedly in the I and HI groups after the start of experimental protocols, although there were no statistical differences between the I and HI groups. Animals were killed at 24 h after experiments and then subjected to pathologic examination. Damaged neurons with shrunken cell bodies and nuclear changes were found on light microscopic examination, mainly in the pyramidal cell layer of the subiculum and cornu ammonis 1. The numerical density of damaged neurons was significantly higher in the HI group than those in the H or I groups (p < 0.05). These damaged neurons were positive on DNA nick end labeling. A DNA ladder was detected on electrophoresis with a DNA sample extracted from hippocampal tissue in the HI group, but not in the other two groups. On electron microscopic examination, not only condensation of the nucleus but also disruption of mitochondria and the cell membrane were detected. These results suggested that hypocapnia under hypotension might cause neuronal cell death in the hippocampus of neonatal rabbit. Not only ischemia but also a metabolic change induced by hypocapnia might contribute to this apoptotic neuronal cell damage.