To reveal the relationship between teratogenesis and carcinogenesis, the author studied brain blastomogenesis features against the background of the development of deformities induced by the combined transplacental effect of NMU and NEU. To induce brain defects such as microcephaly NMU was injected on the 15th day, whereas to induce cerebellar defects- on the 21st day of embryogenesis. Moreover, at the 13th or 17th day NEU was additionally injected, which is found to be highly effective for inducing brain tumors. It was found that in NMU exposure (at the 15th day) until NEU exposure (at the 17th day of embryogenesis) no reliable decrease in brain tumor occurrence was noted, compared with that if only NEU was employed. In the reverse sequence, i. e. first the exposure to NEU (at the 13th day) and then to NMU (at the 15th day) the occurrence of tumors located in cerebral hemisphers was 3 times less. It is assumed that cytotoxic effect of NMU leading to microcephaly is likely to cause the death of a considerable amount of cell population previously transformed.