Life table data were obtained on 2400 ddY/SLC female mice exposed at 10 weeks of age to a single dose of X-rays. Doses given to the whole body were 95, 190, 380 and 570 rad, and to head, trunk or lower body, 190, 380, 570 and 760 rad. The mean survival time of unirradiated controls was 73 weeks. The mean survival times after whole body exposure decreased linearly with increasing dose. The estimate of life shortening per 100 rad was 7.2 per cent for whole body exposure. The life shortening after head or lower body exposure to a dose of 190 rad was 6.2 and 7.1 weeks respectively, and almost no further life shortening up to 760 rad. After trunk irradiation with 190 rad, life shortening was 10.4 weeks and beyond that dose life shortening was 1.1 per cent per 100 rad.