Both underproduction and overproduction of nitric oxide (NO) could lead to various eye diseases. It is known that endothelial NO synthase (eNOS) and neuronal NOS (nNOS) are activated in normal tissues to produce NO for physiological functions. Thus, underproduction of NO results in various eye diseases which could be corrected by providing NOS substrates or NO donors to lower the intraocular pressure, increase ocular blood flow, relax ciliary muscle, etc. On the other hand, immunological NOS (iNOS) is inducible only in pathological conditions by endotoxins, inflammation, and certain cytokines, such as interleukin-1 (IL-1), IL-6, TNF (tumor necrosis factor) and the like. Once induced, iNOS will produce large amounts of NO for long periods of time, so that NO is converted into NO2, nitrite, peroxynitrite and free radicals to induce pathophysiological actions, such as optic nerve degeneration and posterior retinal degeneration lesion, which lead to glaucoma, retinopathy, age-related macular degeneration (AMD), myopia, cataracts and uveitis. To treat/prevent these eye diseases, inhibitors of iNOS activity and/or iNOS induction could be tried.