This report presents a description of the onset of puberty, decline of reproductive function and life span in the Japanese mouse, Mus molossinus Temminck and Schlegel. The summary is as follows 1. In eighty seven female mice the age of the opening of vagina ranges from 35 to 76 days with the median at 64.18 ± 0.445 days. 2. In seventy seven male mice the testes descend into scrota from 16 to 45 days with the median at 25.43 ± 0.405 days after birth. 3. At 65 days after birth mature ovarian follicles are seen in ovary. The metamorphosis in the semiferous tubules at the age of 25 days, are found for the first time in the tubules and at the age of 40 days the spermatozoa are observed in the epididymis first. 4. Decline of reproductive function seems to occur at about 600 days after birth. 5. The life span of the mice fed in captivity is assumed to be 705.85 ± 16.212 days on the average in the breeding room where the temperature is not only hot in summer but also cold in winter.