Female rats were treated with buformin, phenytoin, polypeptide pineal extract, L-DOPA or buformin combined with L-DOPA during 3 weeks before intravenous injections of DMBA (1.5 mg 6 times with one-week intervals) over a period of carcinogen injections and after it till the animals' death. The overall tumour incidence in the control group was 97%, while in buformin, phenytoin, pineal extract, L-DOPA and buformin + L-DOPA treated groups it amounted to 55, 71, 80, 50 and 62%, respectively (P < 0.05). The incidence of mammary adenocarcinoma amounted to 81, 36, 55, 26, 25 and 19%, respectively (P < 0.05). The mechanisms of the similar effects the drugs belonging to different classes produce on chemical carcinogenesis are discussed.