Stress has long been thought of to be associated with increased risk of cancer. Chronic stress is associated with elevated levels of sympathetic neurotransmitter (norepinephrine and neuropeptide Y: NPY) release and immunosuppression. The expression of NPY receptors has been reported in human breast carcinomas. Recently, activation of the NPY Y5 receptor was shown to stimulate cell growth and increase migration in human breast cancer cells; however the effects of NPY have yet to be investigated in a murine model of breast cancer. Thus, the specific aims of the current study were to: (
i) characterize NPY receptor expression in 4T1 breast cancer cells and orthotopic tumors grown in BALB/c mice and ( ii) investigate the impact of NPY receptor activation on 4T1 cell proliferation and migration in vitro. Positive expression of NPY receptors (Y1R, Y2R and Y5R) was observed in cells and tumor tissue. As well, NPY treatment of 4T1 cells promoted a concentration‐dependent increase in proliferation, through increased phosphorylation of ERK 1/2. Using NPY receptor antagonists (Y1R:BIBP3226, Y2R:BIIE0246 and Y5R:L‐152,804), we found the proliferative response to be Y5R mediated. Additionally, NPY increased chemotaxis through Y2R and Y5R activation. These data are in congruence with those from human cell lines and highlight the 4T1 cell line as a translatable model of breast cancer in which the effects of NPY can be studied in an immunocompetent system.