The aim of this paper is to observe whether estrogenic hormones are carcinogen and synergistic effects appear when ,γ, ray and estrogenic hormones are used simultaneously.,One hundred and eighty one female wistar rats were performed an operation on subtotal uterectomy at the upper one third of fused uterus (cervix) 17 days before beginning of the experiment, and then were divided into 4 groups with 40~46 rats in each group as follows:,Group 1: whole-body ,γ, ray exposure was given with 300 rads, then estradiol valerate (E, 10 mg/kg, bw) and hydroxyprogesterone caproate (P, 1250 mg/kg, bw) were injected intramuscularly one day following ,γ, ray exposure and once with interval of 2 weeks for 24 times.,Group 2: whole-body ,γ, ray exposure was given only with the same dose as that in group 1.,Group 3: EP was injected only with the same dose as that in group 1.,Group 4: Control group without any treatment.,Some results were obtained as follows:,Incidences of tumours in group 1~4 were 61.5%, 90.9%, 30.8%, and 10.3%respectively, there were significant or very significant differences in group l~3 compared with control group (p<0.05 or <0.01) . Average latent periods of tumours in the four groups were 208~404 days.,From the point of view of tumour induction concerned, in group 1 and 2, although most of them derived from pituitary gland and mammary glands, but quite different types of tumour observed.,In group 2, the main type of tumours was benign fibroadenomas of mammary glands, with the ratio of malignant to benign tumours being 0.5. However in group 1, the main type of mammary tumours was malignant, with the ratio of malignant to benign tumours being 5.5; and furthermore, the incidence of pituitary gland tumours was much higher than that in group 2 or 3 (46.2%, 6.1%, and 26.9% respectively) .,Besides tumours, there were a number of pyometra in group 1~3, especially in group 1 and 3 with very high incidences of 78.6% and 81.4% respectively.