Carcinogenicity of a single administration of N-nitrosomethylurea: a comparison between newborn and 5-week-old mice and rats.

N-Nitrosomethylurea (NMUrea) was given as a single intraperitoneal injection either to newborn or to 5-week-old (C57BL * C3Hf)F(1) mice and Wistar rats. Newborn mice were more susceptible than 5-week-old mice to the development of lymphosarcomas, lung adenomas and hepatomas, whereas newborn rats were more susceptible than their weaned counterparts to the development of renal anaplastic tumours. Other tumours occured with the same frequency in newborn and mature animals. Tumours of the forestomach in mice were more frequenty found in animals treated at 5 weeks than in those treated at birth. Since NMUrea persists for only a very short time and breaks down spontaneously it seems that the paucity of enzymes related to immaturity in newborns is not a major factor in determining the different susceptibility of newborn animals to NMUrea carcinogenicity.