Erythropoietin (Epo) is known to be a lineage specific cytokine which regulates the number of circulating erythrocytes. Most of it is produced in the kidney. Recently, Epo has been reported to be synthesized in the normal brain, placenta, and capillary endothelium. We also have found that uterine endometrium expresses Epo signals in an estrogen-dependent manner, and that Epo contributes to angiogenesis in the endometrium in mice. To clarify the functional activity of Epo in human reproductive organs, we examined Epo signaling in these organs by Southern analysis of RT-PCR products and studied the distribution of substances relevant to Epo signal transduction by immunohistochemistry and Western blotting. Epo mRNA is expressed in the normal human cervix, endometrium and ovary, but it is not always detected in the specimens. Immunohistochemical analysis revealed Epo-receptor (EpoR) protein in: a) the endothelium of vessels, in glandular and surface epithelial cells, in decidual cells of the endometrium, and b) in follicles at various stages including oocytes, granulosa, theca interna cells and lutein cells of the ovary. Moreover, co-expression of JAK2 and phosphotyrosine, which reflects tyrosine phosphorylation via JAK2, and co-expression of EpoR and STAT5, which is a transcriptional factor relevant to mitogenic activity, were seen at these Epo-responsive sites. Western blotting analysis of these organs confirmed the immunohistochemical results. These findings imply that female reproductive organs can produce Epo, and that signal transduction of Epo contributes to the cyclic changes in the female reproductive organs.