When 31 female Sprague-Dawley rats were exposed to a single total-body dose of 400 r of 250 kvp X-rays on the fortieth day of age, 24 of the rats so exposed developed 43 mammary neoplasms within a 1-year period. Of 31 littermate control rats studied during the same period, only 3 rats exhibited single mammary neoplasms. Thus the irradiated female Sprague-Dawley rat develops a relatively high incidence of rapidly appearing mammary neoplasms as compared to non-irradiated rats, although the mammary neoplastic response to irradiation is subject to hormonal modification. This suggests that radiation-induced neoplasia, of this type in this strain of rat, is at least a two-stage process consisting of stage one[long dash]the induction phase, i.e., radiation injury[long dash]and stage two[long dash]the promoting phase, i.e., the hormonal influence to which the radiation injured tissue is subjected. The total incidence of neoplasia of the pituitary, thyroid, and adrenal glands and of the ovaries did not exceed 6% in the exposed animals, as compared to an absence of these neoplasms in the nonexposed animals. The numbers of these neoplasms were too small to permit meaningful conclusions to be drawn as to the effects of changes in thyroid function, ovarian function, or pregnancy and lactation superimposed on radiation exposure on neoplasia of the endocrine system.