We previously reported significant increases in body weight in B6C3F1 mice continuously exposed to low-dose-rate (21 mGy/day) γ rays compared to that of nonirradiated control mice (Tanaka et al. , Radiat. Res. 167 , 417–437, 2007). To further study the underlying cause of the increase in body weight, feed consumption, adipose tissue weight, liver and serum lipid contents, and selected factors related to glucose and lipid metabolism such as serum levels of insulin and adipocytokines were examined in female B6C3F1 mice irradiated continuously with γ rays at 20 mGy/day in group-housed or individually housed rearing conditions. Increased body weight, adipose tissue weight, serum levels of leptin, and lipid contents of the liver and serum were observed in both group-housed (accumulated dose = 6 Gy, 43 weeks from start of irradiation) and individually housed (accumulated dose = 4.4 Gy, 31 weeks from start of irradiation) irradiated mice compared to nonirradiated controls. Feed consumption measurements, however, revealed no significant difference between irradiated mice and nonirradiated controls when mice were housed individually. Our results show for the first time that the increase in the body weight of mice continuously irradiated with low-dose-rate γ rays is due to adiposity with no corresponding increase in feed consumption.