During the last decade, much attention has centered on melatonin, which was considered to be only a hormone of the pineal gland for many years. As soon as highly sensitive antibodies to indolealkylamines became available, melatonin was identified not only in the pineal gland, but also in extrapineal tissues and cells, e.g. the retina, Harderian gland, gut mucosa, cerebellum, airway epithelium, liver, kidney, adrenals, pancreas, thyroid gland and thymus. Also, melatonin has been found in some non-endocrine cells, e.g. mast cells, natural killer cells, eosinophilic leukocytes, platelets and endothelial and other cells. Functionally, melatonin-producing cells are part and parcel of the diffuse neuroendocrine system as a universal system of response, control and organism protection. The influence of aging upon melatonin-synthesizing cells in the gut is summarized. Melatonin synthesis in the visual system and its hormone level, similarly to the pineal gland, reflect the interaction between the circadian clock and the photic environment. Since retinal melatonin does not contribute to circulating levels and melatonin receptors are present in the retina, its effects on the visual system are primarily mediated by a paracrine mechanism. Although the physiological role of melatonin in the visual system is not exactly determined yet, it apparently includes regulation of gene expression, visual sensitivity and the protection of ocular structures from oxidative damage.