Interesting and still rather controversial attempts are made to define precisely the interrelationships between pineal gland and the development of neoplastic process. Pineal gland is sometimes called an oncostatic organ. There are many reports concerning the disturbances of this gland function in patients with neoplastic diseases. The mechanism in which melatonin (MEL) exerts its oncostatic effect is not clear. Several hypotheses were formulated. In the paper, the following hypotheses were presented: 1. indirect oncostatic MEL influence by endocrine system. 2. immunostimulating action of MEL. 3. mutual influence of pineal gland and endogenous opioid system. 4. direct antiproliferative action on the growth of neoplastic cells of MEL. 5. MEL effect on the action and/or release of growth factors stimulating the growth of neoplastic cells.

Keywords: neoplasm, Melatonin, Pineal Gland