We have examined aspects of the fine structure of the basal laminae associated with the luminal and glandular epithelium and small blood vessels in the human endometrium. Four short studies are presented and reviewed. Study 1 examined biopsies from 20 fertile women taken on days after the luteinizing hormone surge (LH): LH +2, 4, 6, 8 and 10. The basal lamina (both lamina densa and lucida) increased in thickness over the period studied. Study 2 again studied the glandular epithelium and examined the effect of RU486 (a progesterone receptor blocker) administered on day LH +3 and biopsied on day LH +6. The basal laminae were found to be the same as LH +2 control group but thinner than LH +6 control. Study 3 documented increased thickness of the basal laminae between LH +6, 8 and 13 in the luminal epithelium. The within-group coefficient of variation was 16% and 27% for LH +6 and LH +13 groups but only 2 % for LH +8. Study 4 demonstrated an increase in basal lamina thickness associated with small blood vessels between LH +6 and LH +10 in normal fertile women. The basal lamina provides the interface between epithelial and mesenchymal environments; changes in its structure can alter the phenotypic expression of the epithelia. It is one of the maternal barriers that must be transgressed by the trophoblast during implantation. Together, these combined studies provide quantitative baseline structural information on the electron microscopical appearance of the basal lamina during the luteal phase of the menstrual cycle.