The stromal cell populations in the normal human endometrium during the secretory phase of the menstrual cycle will be analyzed through electron microscopy and cytochemistry and compared with previously described stromal differentiation in early gestation (6-12 weeks). Emphasis will be placed on characterization of the predecidual cell and the origin of endometrial granular cells. Nuclear differentiation in the luminal epithelium of the immature rat uterus will be analyzed by transmission, high voltage electron microscopy, and biochemistry to determine the shape, number per nucleus, and chemical composition of the nuclear bodies that arise in relation to estrogenic stimulation. The stem cells of the uterine epithelia in the cycling and immature rat uterus will be identified by following 3H-thymidine labeled dividing and differentiating cell populations. Cellular aspects of uterine regression of experimentally induced deciduomata in pseudopregnant rats will be followed cytochemically and ultrastructurally for comparison with normal postpartum regression.