NO is a chemical messenger with diverse functions throughout the body depending on its enzymatic source and tissue localization.

In the brain and peripheral nervous system, where NOS1 is largely present, NO displays many properties of a neurotransmitter and may be involved in long term potentiation.

It is implicated in neurotoxicity associated with stroke and neurodegenerative diseases, neural regulation of smooth muscle, including peristalsis and sphincter relaxation, and penile erection.

NO is also responsible for endothelium-derived relaxing factor activity regulating blood pressure as produced from its related enzyme NOS3.

Various pharmacological inhibitors of NO synthases (NOS) block these effects, but further distinction of their function has been elucidated by animal models in which these specific genes have been inactivated.