The results of two experiments on the transzygotic transfer of the nitrosoethylurea (NEU) carcinogenic effect are presented. In the first experiment female rats received 60 mg/kg b. w. of NEU at 16, 18 and 20th day of the pregnancy. Their F1 descendants developed multiple tumours of the nervous system after a short latency (up to 40 weeks). The rats of F2, F3, F4 and F5 generations which were not in a direct contact with NEU developed a statistically higher incidence of tumours occurring spontaneously (those of mammary gland, pituitary, and haematopoietic tissue). However, the incidence of nervous system tumours was at the level of controls. In the second experiment male rats received 80 mg/kg b. w. of NEU and then were mated with untreated females. The incidence of the nervous system tumours in the descendants of treated males was somewhat higher than in the controls. The manifestations and mechanisms of the transzygotic effect are discussed.