Endometrial tissue was taken from 21 normal fertile women (aged 18-40 years) between 4 and 13 days after the luteinizing hormone (LH) surge. Systematic random samples of luminal epithelium were taken for both light and electron microscopy and examined morphometrically. Throughout the luteal phase there were remarkably few changes in the volume fraction of nucleus, mitochondria, rough endoplasmic reticulum and 'vesicular system' to cell. Nuclear profile dimensions and cell height also did not change over time. Cell and organelle volume (estimated as volume weighted mean volume) did not change significantly, but showed numerically smallest values on day LH + 13. However the ratio of desmosomes to whole cell and both arithmetic mean thickness and harmonic mean thickness of basement membrane were minimal at the time when implantation would be most likely to occur, i.e. approximately 6 days after the LH peak. Therefore it appears that while some morphometric parameters in human luminal epithelial cells change little during the luteal phase, specific cellular changes occur to the basement membrane and desmosomes which may facilitate embryo implantation. These changes occurred around day LH+ 6 and may be a morphological representation of the 'implantation window'.