Nitric Oxide (NO) an endogenous free radical, has been shown recently to mediate several important biological effects. It plays a neuro-transmitter like role in vascular endothelium, a scond-messenger role in N-methyl-D-aspartate (NMDA) responsive neurons in the central nervous system (CNS), a neurotoxic role after its release from these neurons, and a cytotoxic role after its release by macrophages. This volume reviews among other topics the basic chemistry and physical properties of S-nitrosothiols (RS-NO) and their biochemical mechanisms of action, NO synthase isozymes, NO synthase structure, mechanisms of NO synthesis, regulation of NOS expression and posttranslational modification, and mechanisms involving NO of CNS's damage in virus infections.