The detection of proenkephalin A (PEA) mRNA and encoded peptides in various regions of the female reproductive system raised the possibility that opioid peptides might act as local regulators within this system. Assignment of a specific role for locally synthesized enkephalins has been hampered, however, by the unknown identity of the cells that produce PEA. Using in situ hybridization analysis we have now identified the cell types that express PEA mRNA in the reproductive system of female mice. In the ovary, PEA mRNA was localized primarily in theca cells of preovulatory follicles, and to a lesser extent, in follicular granulosa cells. In the oviducts, where PEA mRNA is most abundant, expression was confined to the secretory and ciliated epithelium of the mucosa. In the uterus, the site of PEA mRNA expression was the deep glandular layer of the endometrium. When pregnancy ensues, and upon decidual transformation, PEA expression by the same uterine cells was dramatically elevated. Elevated levels of PEA mRNA were detected predominantly in the vicinity of the implantation site, suggesting that signaling by the implanted embryo play a role in stimulating PEA expression. Based on these results, possible physiological roles for PEA-encoded peptides as autocrine/paracrine regulators within the female reproductive system are suggested.