Hormones are involved in the regulation of intercellular communication, and gap junction intercellular communication may play an important role in the prevention of endometrial cancer. We have investigated changes in the expression of the gap junction proteins connexin 26 (Cx26) and Cx32 in human endometrial glandular epithelium during the reproductive cycle as well as the influence of hormone replacement therapy. Frozen sections from 71 endometrial tissue samples (53 taken from women who had undergone hysterectomy during the menstrual cycle, 3 early pregnancy deciduae and 15 from menopausal women, some of whom were receiving estrogen alone or estrogen plus progesterone) were analyzed by immunofluorescence and confocal laser scanning microscopy. Cx26 and Cx32 were expressed weakly in the proliferative phase, markedly during ovulation and most strongly in the mid-secretory phase; by the late secretory phase, they decreased drastically. Cx26 and Cx32 also were expressed in early pregnancy. Women who had received estrogen and progesterone expressed the Cxs, but those who had received estrogen only or no therapy did not. These results were confirmed by Western blot analysis. They indicate that expression of Cx26 and Cx32 is correlated with cell differentiation and with the glandular function of the endometrial epithelium and suggest that expression of Cxs is controlled by serum progesterone.