Abstract: Objective:To determine the expression of the full-length (NK 1R-FL) and truncated (NK1R-Tr) neurokinin-1 receptor (NK 1R) and the neurokinin-2 receptor (NK 2R) in breast cancer tissues and cell lines, as well as to study the effects of the NK1R and NK2R antagonists on the growth of breast cancer cells. Methods: Immunohistochemistry and Western blot assays were used to detect NK1R, NK1R-FL, and NK 2R expression in clinical samples of primary breast cancer tissue, benign lesions, and normal breast tissue, as well as in different breast cancer cell lines. Cell proliferation and soft agar growth tests were performed on cells treated with the NK1R and NK2R antagonists to study the ectopic overexpression of NK 1R-FL and NK1R-Tr in breast cancer cell lines. Results:Total NK 1R expression was detected in the breast cancer tissues, benign lesions, and normal breast tissues. Compared with the normal breast epithe -lia and benign breast lesions, the expression levels of NK 1R-FL and NK2R decreased in the carcinoma. These changes were also relat-ed to the carcinoma type, histological grade, lymph node metastasis, HER 2 and Ki-67expression, and estrogen and progesterone recep -tors in breast cancer. The expression levels of NK 1R-FL and NK 2R were high in the HBL-100 breast cell lines of para-neoplastic tis-sues, but NK1R-Tr expression was low. The MDA-MB- 231, T- 47D, and MCF-7 cells only expressed NK 1R-Tr. NK 1R-Tr or NK 1R-FL overexpression caused the decreased inhibition rate or increased levels of the NK1R and NK2R antagonists in the breast cancer cells. Conclusion:NK1R-FL and NK2R are co-expressed in normal cells. NK 1R-Tr is highly expressed in breast cancer cells and exerts negative feedback to regulate NK 1R-FL and NK2R expression in all cells, especially cancer cells.