Endometrial biopsies were obtained from fertile women, aged 20-40 years, with regular cycles of 25-35 days. Chronological dating of the material was carried out by determination of the luteinizing hormone (LH) peak by daily LH assay. Tissue was taken from three or four sites along the biopsy and processed for electron microscopy. Half micron thick sections were analysed by light microscopy and a morphometric examination of the glandular epithelium carried out. Nuclear size was assessed using an unbiased measuring technique and the volume density of the nucleus in the cell was also measured. A series of 12 women was examined, four at each of days LH + 2, 4 and 5. An attempt was made to control a variety of factors which contribute to the variability in the histological dating of the endometrial biopsy. In doing so it was possible to analyse the relative contributions of each sampling level to the overall variance. In contrast to earlier studies there is a relatively small intersubject variation with a correspondingly large within-biopsy variation. These results may indicate that the cellular events in the glandular epithelium, between ovulation and the middle of the luteal phase, are precisely regulated.