Melatonin is a product of the pineal gland. Synthesis and release of this hormone is inhibited by light. The biological activity of melatonin is associated with its receptors--ML1 and ML2. Melatonin plays a role in the biologic regulation of circadian rhythms, sleep, mood, reproduction, tumor growth and aging. It may also modulate the activity of various receptors in cancer cells. The hormone is a free radical scavenger, an antioxidant and immunomodulatory agent. Antioxidant properties of melatonin are connected with its neuroprotective activity in several degenerative disorders. The etiology of the neurodegenerative diseases which are characterized by the progressive and irreversible destruction of specific neuronal populations is complex and multifactorial. One of causes of neurodegenerative damage in the nervous system is oxidative injury, which results from an inbalance between free radical formation and antioxidative mechanisms. The efficacy of melatonin in the inhibition of the oxidative stress was estimated in various neurodegenerative disorders whose pathogenesis is associated with cytotoxic activity of free oxygen radicals, such as Alzheimer's or Parkinson's disease. Melatonin may have a clinical potential for the treatment of neurodegenerative disorders in the central as well as peripheral nervous system. (Ref. 38.)