In the experiments with transplacental exposure of mice to N-nitrosoethyl urea (NEU) an enhanced carcinogenesis was noted not only in the first but also in the second generation. This effect in mice of the second generation was manifested in earlier development, as compared with control animals, of different neoplasms as well as in somewhat increased frequency of their detection. Postnatal exposure to NEU would enhance carcinogenesis in mice of both experimental and control groups, whereas no enhancement of carcinogenesis was observed in animals exposed to NEU in progenesis, compared with control mice. Postnatal x-ray irradiation also failed to produce enhanced carcinogenesis, compared with control animals i.e. those exposed to NEU in progenesis. While in female mice of the same group, contrary to the control, ovarian tumors under the effect of irradiation did not arise. It is suggested that there is a pathogenic connection between the inheritance of carcinogenic effect by oocytes and the loss by the ovary its capacity to produce neoplasms in response to irradiation.