Mammary tumors are the third most common tumor in cats after cutaneous neoplasia and lymphosarcoma. More than 80% of feline mammary tumors are adenocarcinomas, which are similar to human mammary adenocarcinomas in their biologic behavior and histologic characteristics. Despite treatment with radical mastectomy, feline mammary adenocarcinoma is associated with a high incidence of metastasis to regional lymph nodes, spleen, liver and lungs. Prolactin (PRL), a protein hormone synthesized by the anterior pituitary gland, stimulates mammary epithelial cell proliferation and differentiation. In many mammalian species, PRL is also produced within the mammary gland. In humans and rodents, PRL is an important mitogen for mammary neoplasia. However, the role of PRL has not been investigated in feline mammary adenomas and adenocarcinomas. The purpose of this study was to determine if feline mammary tumors expressed PRL. Formalin-fixed paraffin-embedded sections of archived mammary tissues (n=6) previously submitted to the Oregon State University Veterinary Diagnostic Laboratory were cut with a width of 6 microns and studied by a modified indirect immunohistochemistry protocol with a polyclonal PRL antibody. Feline anterior pituitary tissue was used to verify the validity of the procedure as PRL expression by lactotrophs in the tissue had positive and specific staining. Prolactin expression was positive and specific in glandular epithelial cells of the mammary gland of some cats (2 of 4) with adenocarcinoma. In conclusion, these results suggest a contribution of PRL in the development of feline mammary neoplasms. Further exploration into the relationship between PRL and feline mammary adenocarcinomas is needed.