Estriol, 0.5 mg, was administered intravaginally as ovula once a day for 16 days to 5 menopausal women. Endometrial biopsies were taken before and after the last day of treatment. Before the treatment scanning electron microscopy showed a typical menopausal uterine surface with sparsely occurring ciliated cells among the non-ciliated ones. The non-ciliated cells possessed a small number of short microvilli. After estriol treatment the ciliated cells occurred more frequently. The cells possessed more numerous and longer microvilli and several non-ciliated cells showed apical protrusions. These changes are considered to be signs of an estrogenic influence. It is concluded that the estriol release from the vaginal ovula is sufficient to induce an estrogenic response in the surface of the uterine epithelium.