Caspases are believed to play a key role in the delayed neuronal cell death observed in the rat brain after hypoxic-ischemic (HI) insult. Caspase inhibitors have been developed as antiapoptotic agents. Hippocampal damage after HI insult is strongly related to tissue temperature, and systemic hypothermia has been introduced clinically for brain protection. In this study, we examined the effects of a caspase inhibitor and systemic hypothermia on neuronal protection in the developing rat brain. Postnatal d 7 rat pups were subjected to the Rice model of hypoxia for 1 h. Systemic hypothermia was induced with a water bath at 29 degrees C. Before HI insult, a pan-caspase inhibitor, boc-aspartyl-(OMe)-fluoromethyl-ketone (BAF), was injected into the cerebral ventricle. The ipsilateral hippocampus was subjected to caspase assays and histologic assessment. The HI group at 37 degrees C (HI-37 degrees C) showed a peak of caspase-3 activity 16 h after insult. This activity was significantly reduced in the presence of BAF or hypothermia (HI-29 degrees C group, p < 0.05) or by the combination of HI-29 degrees C + BAF (p < 0.01 versus HI-37 degrees C). The number of neuronal cells in the ipsilateral hippocampal CA1 region in the HI-37 degrees C group was significantly decreased (62.9% versus control). The number of neuronal cells was maintained in the HI-37 degrees C + BAF group (82.7%), the HI-29 degrees C group (78.7%), and the combination group (95.2%) (p < 0.05 versus HI-37 degrees C). A combination of systemic hypothermia and BAF produced a strong protective effect against neuronal damage in the developing rat brain, along with a reduction in caspase-3 activity.