The primary cancer chemoprevention is an important topic of experimental oncology. We have analyzed the possible oncostatic properties ofmelatonin in a combined model of radiation plus chemocarcinogen-induced mammary carcinogenesis. Virgin female rats of Wistar:Han strain were continuously irradiated with daily dose 96 mGy of gamma rays up to 15 days. At the end of irradiation, between 52-60 postnatal days, 7,12-dimethylbenz(a)anthracene was administered by gavage, in three 10 mg/rat consecutive doses. A part of animals drank melatonin in a concentration 100 microg/ml of tap water, continuously from the beginning of irradiation and 26 weeks after its end. The aim of the experiment was to investigate the preventive effect of melatonin on mammary tumor patterns. Relatively low incidence of mammary tumors in the noninfluenced group was probably connected with generally very low sensitivity of Wistar:Han female rats to single dose of chemocarcinogen in mammary carcinogenesis induction. In our trial melatonin decreased markedly the volume of mammary tumors, but did not influence any other tumor characteristics. The chemopreventive effect of melatonin, derived from in vivo realized mammary carcinogenesis study in female Wistar:Han rats was limited. The cancer preventive properties of melatonin should be investigated in the future especially from the standpoint of susceptible strain, effective doses, and mode plus sufficient length of application.