The roles of age at treatment and dose in carcinogenesis in C3Hf/Dp mice with a single administration of N-nitroso-N-methylurea.

C3Hf/Dp mice were given a single i.p. injection of 50, 25 or 5 mug/g N-Nitroso-N-Methylurea (NMU) at either 1 or 70 days of age or 50 mug/g at 21 days of age. They were observed until death or until 120 weeks of age. The two highest doses of NMU produced tumours in a wide spectrum of organs, including the thymus, forestomach, lung, liver (only in males), kidneys, ovaries and orbital glands. The only two tumour types which appeared to be closely related to the occurrence of death were thymic lymphomata (most of which were found in mice dying before 40 weeks after treatment) and carcinomata of the forestomach. Lifetime analyses are presented concerning the occurrence of these two tumour types as well as the occurrence of any tumour after 40 weeks of age or since treatment. Incidences of thymic lymphomata were 67.6%, 39.0% and 21.2% in mice receiving 50 mug/g NMU at 1, 21 and 70 days respectively and 17.1% in mice receiving 25 mug/g at 1 day. In the other groups the incidence of thymic lymphomata was zero or negligible. The rate of progression of thymic lymphomata until death was related to both earliness of treatment and dose. On the contrary, incidences and progression of carcinomata of the forestomach were unrelated to age at treatment. Since breakdown of NMU is very rapid and does not require enzymes, these results are considered as evidence that host-tumour interaction differs from organ to organ. No excess of tumours over the controls was found in mice receiving 5 mug/g either at 1 or 70 days of age.