Recent evidence indicates that oxytocin (OT), in addition to the induction of myometrial and myoepithelial cell contraction, can influence proliferation and differentiation in developing mammary glands and in breast cancer cells, hence the interest in detecting and locating OT receptors (OTRs). We produced rabbit antisera and a monoclonal antibody against a synthetic peptide corresponding to the carboxy terminus of the predicted OTR sequence. We tested their specificity in immunoblasts and immunocytochemical tests. All of the antibodies specifically stained myometrium (at term of pregnancy). In the human breast, OTRs were detected in myoepithelial cells along ducts of normal lobules and in sclerosing adenosis. Intraductal cells in benign hyperplastic lesions were also positive. OTRs were demonstrated in cases of primary and metastatic carcinomas of the breast. In the same tissues, OTR gene expression was shown by reverse transcriptase polymerase chain reaction procedures detecting the specific mRNA. These results suggest that the interaction between OT and its receptors might play a role in the origin and evolution of non-neoplastic lesions and carcinomas of the breast.