The transplacental activity of N-ethyl-N-nitrosourea (ENU) was tested in rats, rabbits, Syrian golden hamsters, Dzungarian dwarf hamsters, guinea pigs, dogs, and rhesus monkeys. In the offspring of rats, multiple tumours of the central and peripheral nervous system were found in 100 per cent, whereas in rabbits kidney tumours developed. In Syrian golden hamsters, the application led mostly to neoplasms of the thyroid gland. Dzungarian dwarf hamsters and monkeys prooved to be resistant. 9 out of 15 dogs died shortly after birth. In two dogs a carcinoma of the thyreoid gland and one of an ovary were found. Because of an infection, the experiments with guinea pigs were to be interrupted untimely. Therefore, a final answer is impossible. Own results were compared with the findings of other authors. The significance of the different susceptibility in distinct species is discussed.