The authors demonstrated the effect of deuterium depleted water (DDW) with a concentration of 60 ppm to be a very good inhibitor of the neoplastic cell proliferation in outbred Wistar rats inoculated with two strains of the highest tumor aggressiveness (Walker 256 and T8 Guérin). For this experiment were organized groups, in equal parts by sex of outbread Wistar rats, with an average weight of 100g, which were established both control groups and experimental groups. Through the experiment, animals were monitored in anatomoclinical terms. Laboratory tests were executed periodically, especially cytomorphological. Dead animals were subjected to a careful pathological examination. It is well known that high malignant Walker 256 and T8 Guérin tumour strains develop a solid, ulcerated subcutaneous cancer, after an incubation of 5-6 days. The reproductibility of this type of cancer is of 95%. Therefore the death of the rats occurs within a short period of 40-60 days. The cumulative effect of DDW 60 ppm on the rats grafted with Walker 256 and T8 Guérin strains was about 28-30 %. This percentage comprises both the animals in which the effect of primary reject of the tumour graft was noticed and the healing effect after an important development of the tumour.