166 Sprague-Dawley Rats (148 males and 118 females) submitted to different hormonal conditions were exposed to 3 repeated whole-body irradiations of 14.8 MeV neutrons or sham-irradiated between 50 and 65 days of age (total absorbed doses: 3 x 2 rad and 3 x 8 rad). They were observed for 11 months. In the male group, a small number of tumors was obtained. In the female group, 75 breast neoplasms were scored in 41 of 78 irradiated animals (54 fibroadenomas, 20 adenocarcinomas and 1 fibrosarcoma). A second group of benign and malignant tumors was observed from 200 days on. The neoplastic response to fast neutron fractionated irradiations was increased by pregnancy with subsequent lactation. Estradiol and progesterone receptors were measured in 34 tumor samples. Fibroadenomas (1;5) and adenocarcinomas (1;3) bound labelled steroids. Like in human breast cancer metastases, steroid receptors are found in recurrences only if present in the primary tumor.