Nitric oxide (NO) is a small, soluble, diffusible, highly reactive gas molecule, which is critical to numerous biological processes, including vasodilation, neurotransmission and macrophage-mediated microorganism and tumor killing. NO often plays a role of a biological signal. It is involved in neuron to neuron signaling, and probably contributes to the formation of memories. Most non-pathological functions of NO are mediated by activation of guanylyl cyclase, or by nitrosylation of proteins. NO can also inhibit oxygen consumption by mitochondria. (NCI)