There are controversies over the influences of thyroid function on carciogenesis and progression of the breast cancer. To reveal the interrelation of the breast cancer and thyroid function in rats, this study was performed by employing N-nitrosobutylurea which is known for its carciogenetic action. Female Wistar rats were divided into 4 groups, namely, group with total thyroidectomy, group receiving thyroid sic (without thyroidectomy), group receiving thyroxin (without thyroidectomy), and control group. The thyroidectomy was done 14 weeks after birth and, thyroid sic and thyroxin started to be administered 14 weeks and 8 weeks after birth, respectively. Thyroid sic and thyroxin were given 0.5mg/day/rat and 10μg/100g body weight for 5 days in a week, respectively. N-nitrosobutylurea of 6mg/day/rat was administered to the rats of all the groups for 5 days in a week from the 6th post-natal week. In patients with various breast diseases functional levels of the thyroid gland were determined by measuring triiodothyroxine (T(3)), Thyroxin-binding-globulin (TBG), thyroid-stimulating-hormone (TSH) and prolactin. Blood samples were obtained in early morning and, serum samples obtained were frozen and stored. T(3), TSH and prolactin were measured by radioimmunoassay, triosorb test by resin-sponge uptake method and TBG by electrophoresis with cellulose acetate membrane. The patients studied consisted of 8 with fibroadenoma mammae, 4 with mastopathy and 52 with breast cancer. Results obtained in the study are as follows: 1) Carciogenetic action of N-nitrosobutylurea in rats with total thyroidectomy was greater at the beginning of the administration, but long-term administration of N-nitrosobutyl reduced its carciogenetic effect. 2) There was no carciogenetic influence in NBU on rats with thyroid sic administration. 3) No carciogenetic effect was found in rats with subcutaneous administration of thyroxin. 4) TSH and PRL were higher in patients with breast cancer, particularly in earlier stage, compared with those in patients with benign breast diseases, including mastopathy and fibroadenoma mammae. However, there was no correlation of TSH and PRL levels to stages of the breast cancer. 5) PRL was elevated after radical mastectomy than that before the operation. 6) There was no significant difference in T(3), triosorb test and TBG between patients with the benign diseases and those with breast cancer.