The pelvic region of random bred female ICR-JCL mice and CF1 mice was irradiated with various doses of X-rays at 1-week intervals to determine the relationship between the X-ray dose and induction of rectal carcinoma. The incidence of rectal carcinoma in ICR mice was zero after a single dose of 2,000 rad of X-rays, but 31% after a single dose of 3,000 rad, 6% after 2 doses of 1,500 rad, 25% after 3 doses of 1,500 rad, 42% after 2 doses of 2,000 rad, and 95% after 3 doses of 2,000 rad. This cancer developed in 70% of CF1 mice exposed to 2 doses of 2,000 rad. No case of this tumor was observed in the control animals not exposed to X-rays. The development of this tumor was found to depend on the X-ray exposure dose. Local X-irradiation of the pelvic region is one of the effective methods for inducing rectal carcinoma in mice. Rectal cancers induced by X-irradiation were adenocarcinoma of the tubular, papillary, and mucinous type, and frequently showed invasive growth into the deep layers of the rectal wall.