The nitrogen cycle initiates direct reduction of N2 to NH3 by enzymatic reactions. We hypothesize that l-dihydroxyphenylalanine (l-DOPA), a catecholamine, could be a source of nitric oxide (NO). In order to determine whether l-DOPA generates NO and induces any biological change in the eye, we measured the generation of NO in vitro and in vivo, and investigated the histopathological changes caused by injection of l-DOPA into the vitreous of rats. We also hypothesized that melanin granules may affect the generation of NO during the metabolism of l-DOPA, since l-DOPA is a precursor of melanin in the brain and the eye. Therefore, we compared the effects of l-DOPA on the generation of NO between amelanotic and melanotic rats. NO was measured as diffusion currents by NO electrodes. In vitro, various concentrations of l-DOPA (5, 29.9, 79.4, 152.7, and 249 μM) were added to the medium. The inhibition of NO generation by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazole-1-oxyl 3-oxide (carboxy-PTIO) was tested. In vivo, NO generation in the vitreous of rats was measured and the eyes were enucleated under anesthesia after l-DOPA injection. The ocular tissues were subjected to histological examination. NO was produced from l-DOPA in a dose-dependent manner and was scavenged by carboxy-PTIO in vitro. NO in the vitreous of melanotic rats was generated from l-DOPA. Histological examination with hematoxylin-eosin staining revealed vasodilation in the ciliary vessels and the choroid after l-DOPA injection. Both effects were greater in melanotic rats than in amelanotic rats. The vasodilation may be attributable to NO as well as to superoxides, which can be regulated by the existence of melanin.