One of the major physiological roles of melatonin is its synchronizative influence on circadian rhythmicity and the induction of seasonal responses to changes in day length. In seasonally reproductive animals like the hamster, melatonin has been established as antigonadotrophic on the gonadal axis. Melatonin has been used successfully to treat jet lag and some circadian-based sleep disorders. It has the immunomodulatory role, which can be supported by the existence of specific binding sites in lymhoid cells, thymus and spleen. Not negligible is the antioxidative effect of melatonin and the ability to scavenge some reactive forms of oxygene. Melatonin has potentially important influence on the neoplastic growth and direct and indirect oncostatic effect in some forms of neoplasia. The beneficial influence of melatonin alone or its combination with immunotherapy, radiotherapy or chemotherapy in many clinical studies in patients with tumors was demonstrated.

        Key words: melatonin, antioxidative effect, therapy of tumors