In the present work, first we reviewed and completed our previous experiments on the antiproliferative effect of oxytocin (OT) in breast cancer cell lines. In vitro, OT 10 nM and 100 nM inhibited cell proliferation of MDA-MB231 (human breast carcinoma) and TS/A (mouse mammary carcinoma) cell lines. In vivo, OT significantly reduced the growth of TS/A mammary tumors. Both effects are mediated by specific receptors (OTR) distributed on cell surface. Second, using immunohistochemistry and RT-PCR we detected OTR and OTR mRNA in normal and pathological breast tissue. There is no correlation among OTR presence in breast carcinomas and the age of patients, tumor stage, estrogen receptor positivity, oncogene expression and proliferation rate of the same tumor. On the contrary, progesterone and OTR expression are correlated. These data confirm our previous evidence of a role of OT and OTR in normal and neoplastic breast cells.