The life spans and spontaneous tumors in a total of 1960 Wistar Mishima (WM/ MsNrs) rats, inbred strain, from the 80-130th generations were examined. The average life span (mean ± SD) was 731 ± 173 days (n=1053) in the males and 813 ± 214 days (n=907) in the females (p<0.0001). The average life span of tumor-afflicted females was significantly longer than that of the non-tumor group (p<0.0001), while no such difference was observed in males. Tumors were observed in 33 males (3.1%) and 246

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... s (27.1%). In the males, tumors were often observed under the skin (2.2%). Frequencies of tumors in lung and liver, bones and intestine were less than 0.5%. In the females, incidence of mammary tumor was 20.1%, and various organs such as ovaries, uterus, bones, lung, and liver had tumor incidence frequencies of less than 3.5%. It was concluded that WM/MsNrs rats might be suitable for life span and age-related studies because of their characteristics of length of longevity and the low incidence of spontaneous tumors in both sexes.