In this study we examined the fine structure of the human glandular epithelium in women with idiopathic ovarian failure in artificial cycles produced by two different types of hormone replacement therapy (oestradiol valerate orally and either intramuscular injection of progesterone or progesterone via vaginal pessary), both of which had been claimed to be capable of supporting successful implantation. Biopsies were performed on day 19 of the artificial cycle. A variety of sterological probes were used to quantify the structural characteristics of these cells and the results obtained were compared with those from normal fertile subjects biopsied on days 2 and 5 after the luteinizing hormone (LH) surge. The glandular epithelial cells from the vaginal group had a less elaborate secretory apparatus when compared with the intramuscular group or with LH + 5 controls. Many of the features of the vaginal group suggested a continued oestrogenic influence, while the intramuscular group seemed to be more advanced than LH + 5.