AbstractThe time course of the hormonally controlled deciliation cycles of the centriolar complexes in the cells of the luminal epithelium of the uterus of the ovariectomized and adrenalectomized rat was analyzed at ultrastructural levels. The results were expressed quantitatively. The half‐life of the solitary cilia after progesterone administration in the cell population was 14.6 hr, longer approximately by 1 hr than that previously reported for the estrogen‐induced deciliation of the same cells.The time course of the progesterone‐induced loss of solitary cilia strongly suggested that the phenomenon is biphasic. After the initial loss of 50%, the process progressed slowly. Twenty‐four hours after the injection of the hormone, 35% of the cilia remained; after 60 hours, 4% were still present. This is in striking contrast to the effect of estrogen, which causes almost complete deciliation within 24 hr after the injection of the hormone. Prolonged exposure of the deciliated luminal epithelial cells to progesterone leads to disarrangement of the diplosome relationship. The phenomenon might be causally correlated with the block of estrogen‐induced mitoses by this hormone; this view, however, needs further experimental corr