During HIE free radicals are generated within mitochondria and also as byproducts in the synthesis of prostaglandins.These free radicals ignite a secondary phase of subsequent damage to the brain by attacking membranal fatty acids. Nitric oxide (NO) is involved in the cascade of metabolic events that contributes to HIE. It mediates, in part, the cytotoxic activity of macrophages, induces relaxation of blood vessels, and also acts as a neurotransmitter in the central and peripheral nervous system. Therefore, the therapeutic value of NO synthase inhibitors, among many other agents used to ameliorate the course of HIE, is currently under investigation in experimental animals.