Melatonin is a derivate of biogenic amine of serotonin identified in all classes of animals including flatworms. Melatonin demonstrates different physiological functions the main of which is circadian rhythm regulation. Via specific G-protein coupled receptors, melatonin affects the target cells changing the levels of other hormones. On early stages of embryonic development, biogenic amines as well as melatonin play a role of specific signal cell molecules that regulate processes of cellular renewal. This work has studied physiological function of melatonin in free-living flatworms, planarian Schmidtea mediterranea. The influence of melatonin on diurnal dynamics of stem cells proliferation was investigated using an immunocytochemical method and confocal laser scanning microscopy. The specific antibodies against H3 phosphohistones were applied for immunocytochemical identification of proliferative cells. It was shown that melatonin (1 µМ) decreased the total number of proliferative cells in planarians. It was also found that the diurnal dynamics of cells proliferation in planarians was changed by melatonin: regular rhythmic oscillations observed in the control group of animals were smoothening. Further researches are required to clarify mechanisms of melatonin actions.