Freeze-fracture replicas were used to study gap junctions in the glandular epithelium of the human endometrium. The junctions were found to be small both in number and size during the early and mid-proliferative phases of the cycle. In the late proliferative phase and later in the cycle, their number and size were significantly increased (p < 0.05). In the late secretory phase, the gap junctions again showed a significant reduction both in number and size (p < 0.05) in comparison with the mid-secretory phase. These findings suggest that the formation and growth of gap junctions might be induced by estrogens, and that they might play a role in the differentiation of the endometrial cells as well as in the coordination of their secretory function.