History, survival, and growth patterns of B6C3F1 mice and F344 rats in the National Cancer Institute Carcinogenesis Testing Program.

The history of, and rationale for, the selection of the hybrid B6C3F1 mouse (C57BL/6 female X C3h/He male) and the inbred F344 rat for National Cancer Institute (NCI) bioassays is described. Survival percentages at the end of 2-year tests and weight-gain patterns during the tests of control animals are presented to guide investigators using these same animals in similar long-term experiments. Because information on a large number of animals (9385 mice and 10,023 rats) from a number of laboratories is presented, the conclusions should serve to give general guidance to investigators holding the same animals under a diversity of husbandry conditions. The program experience has been that the B6C3F1 mouse survival at the end of a 24-month study (25.5 months of age) is 80%; the F344 survival for the same period is 75%. This contrasts with the generally held assumption that rats are longer lived than mice. For the period of time from which animal data were collected, there was a progressively slight decrease overall in survival percentage. This observation cannot be explained, and contravenes the expectation that methodological improvements in producing the animals and marked physical improvements in the testing laboratories should have resulted in improving the survival. Weight gain patterns have a distinct and somewhat similar sexual dimorphism for both rat and mouse. The males of each species grow much faster initially and then essentially level off. The female mouse grows slowly and steadily, and by 2 years of age almost equals the male; the female rat shows the same steady gain, but is much lighter than the male at 2 years of age.