SUMMARY Neonatal brain injuries, hypoxia-ischemia (HI) brain injury is one of the leading causes of disability in children. Mitochondrial dysfunction due to activation of permeability transition pore (mPTP) has been implicated in cell death after HI insult but only in the mature brain. It has been proposed that mPTP is non-specific protons/ions leak across the inner mitochondrial membrane which dissipates proton motive force and renders mitochondria deficient or incapable of ATP production, often driving cellular death in the ischemic brain. However, the strategies to address mPTP in the model of HI brain injury in neonatal mice or rats were unsuccesful. Thus, an identical biophysical process, an activation of mPTP, triggered by similar HI stress contributes to the brain injury in the mature animals and may not be mechanistically significant in the immature brain. We have reasoned and found that biophysical and pharmacological properties of the mPTP activated during HI in neonates differ from that in adult animals. In neonates these mPTP are not cyclophilin D (CypD) dependent. This a) defines neonatal failure of the strategies which are neuroprotective in adults and b) require a different therapeutic approach. Aim 1. To determine if CypD-independent mML contributes to SEF and cellular injury during reperfusion. Aim 2. To determine the pharmacological regulation and structure of CypD- independent mitochondrial membrane leaks. Aim 3. To determine the role of CypD-dependent mPT in neonatal HI brain injury. Our work will provide the very first direct evidence for pathogenic role of mitochondrial permeabilization in neonatal HI brain injury models. This will allow us to better understand its regulation and structure function relationship and help to develop highly efficient strategies of brain protection against hypoxic ischemic stress by direct targeting of the ion-conducting parts of the mPTP.