Abstract: Nitric oxide is synthesized by the enzyme nitric oxide synthase. Nitric oxide synthase maintains the normal levels of nitric oxide in body. Discovery of nitric oxide synthase has lead to a new era in the drug research. Nitric oxide regulates the normal physiological balance and functions of the body. Three isoforms of nitric oxide synthase have been identified, an endothelial type, a neuronal type and a macrophage (inducible) type. Nitric oxide helps in regularizing normal blood pressure, relaxation of smooth muscle, release of growth hormone, insulin and pancreatic polypeptide. Nitric oxide plays very important role in host defense mechanism of body. Studies have shown that excess or deficiency of nitric oxideis associated with many diseases. In spite of the involvement of NO in several physiological functions, overproduction of NO can be harmful. An excess of NO produced by nNOS or iNOS is involved in pathological conditions, including post-ischemic stroke damage, septic shock, seizures, schizophrenia, migraine headaches, Alzheimer`s disease, development of colitis, tissue damage and inflammation, osteoporosis and rheumatoid arthritis, depression, neuroimmune diseases. In this study we have taken review of the important role of nitric oxide in biological system and the strategies to inhibit the overproduction of nitric oxide.