Tanaka, S., Tanaka, I. B. III., Sasagawa, S., Ichinohe, K., Takabatake, T., Matsushita, S., Matsumoto, T., Otsu, H. and Sato, F. No Lengthening of Life Span in Mice Continuously Exposed to Gamma Rays at Very Low Dose Rates. Radiat. Res. 160, 376–379 (2003). Late effects of continuous exposure to ionizing radiation are potential hazards to workers in radiation facilities as well as to the general public. Recently, low-dose-rate and low-dose effects have become a serious concern. Using a total of 4000 mice, we studied the late biological effects of chronic exposure to low-dose-rate radiation as assayed by life span. Two thousand male and 2000 female 8-week-old specific-pathogen-free (SPF) B6C3F1 mice were randomly divided into four groups (one nonirradiated control and three irradiated). Irradiation was carried out for approximately 400 days using 137 Cs γ rays at dose rates of 21 mGy day –1 , 1.1 mGy day –1 and 0.05 mGy day –1 with total doses equivalent to 8000 mGy, 400 mGy and 20 mGy, respectively. All mice were kept under SPF conditions until they died spontaneously. Statistical analyses showed that the life spans of mice of both sexes irradiated with 21 mGy day –1 ( P < 0.0001) and of females irradiated with 1.1 mGy day –1 ( P < 0.05) were significantly shorter than those of the control group. Our results show no evidence of lengthened life span in mice continuously exposed to very low dose rates of γ rays.