This paper reports the electron microscopic appearance of the uterine glandular epithelium in a group of 25 women of known fertility who underwent endometrial biopsies at known times after the luteinizing hormone (LH) surge between LH + 2 and LH + 6. The use of these well-timed endometrial biopsies enables one to gain a clearer insight into the cellular dynamics of the glandular cells, between ovulation and the middle of the luteal phase. Under the influence of progesterone, the cells are transformed from relatively inactive cells full of free ribosomes to very active polarized cells, containing giant mitochondrial profiles, intracellular deposits of glycogen/glycoprotein-rich material and a complex intranuclear channel system. Intimate associations between organelles develop, the most notable being that between a semi-rough endoplasmic reticulum and the mitochondria. By taking biopsies that are accurately timed, relative to the LH surge, from a group of women of known fertility, the sequences of changes in ultrastructure are established with considerably more precision than was previously available.